edit: there seems to have a new error popped up... before it worked as stated below, but now I need to add "Flak" to the flags of the two primaries to make it work again... strange.
Seems I'll have to do a bit more experimenting and testing to find out what went wrong... 
Oh, and thanks to aa51 from the irc-channel for help in testing this again
.
[edit2: Added the Flak-flag to the tbm below, to prevent people from jumping into trouble
.]Sorry in advance for necromancing... but I just did a homing primary, too; just that I simply used spawning for it.
Granted, that way the hits will count as secondary-hits, but I'd say I prefer that over crashes...

I basically simply took the Prometheus and a Rockeye with Prometheus-stats + Prometheus-looks, then somewhat changed some values.
IMPORTANT POINT: For this to work, it seems the "remote detonate" is needed in the parent primary. That was the main problem I had to figure out.
*tested with 3.6.10.5618* (well, only the LR-variant, but technically, that shouldn't matter...)
*Also, I wanted to make the primary homing, and the secondary to inherit it's parent's target... but at that point I still hadn't figured out the remote-detonate was necessary, and again deactivated the homing for testing if it was a problem... as this didn't seem to cause differences, it
should be safe to turn the homing on the parent primary on again.

I just didn't yet get to test it again...

Here's the xxx-wep.tbm-entry:
#Primary Weapons
$Name: Prometheus T
+Title: XSTR("GTW-5 Prometheus Torpedo", -1)
+Description:
XSTR(
"Hull Damage: 77,14/s
Shield Damage: 85,71/s
Subsys Damage: 30/s
Energy usage: 2,86/s
Range: 1500m", -1)
$end_multi_text
+Tech Title: XSTR("GTW-5 Prometheus T", -1)
+Tech Anim: Tech_GTW-5_Prometheus_S
+Tech Description:
XSTR("", 3277)
$end_multi_text
$Model File: none
@Laser Bitmap: laserglow03
@Laser Glow: 2_laserglow03
@Laser Color: 20, 150, 20
@Laser Color2: 20, 150, 20
@Laser Length: 10.0
@Laser Head Radius: 1.0
@Laser Tail Radius: 1.0
$Mass: 0.2
$Velocity: 750.0
$Fire Wait: 0.35
$Damage: 30
$Arm distance: 400
$Detonation Range: 500
; $Blast Force: 200.0
; $Inner Radius: 20.0 ;; radius at which damage is full
; $Outer Radius: 140.0 ;; max radius for attenuated damage
; $Shockwave Speed: 30.0 ;; velocity of shockwave. 0 for none.
;$Dinky shockwave:
; +Shockwave damage:
; +Shockwave damage type:
; +Blast Force: 100.0
; +Inner Radius: 5
; +Outer Radius: 30
; +Shockwave Speed: 6
; +Shockwave Rotation:
; +Shockwave Model:
; +Shockwave Name:
$Armor Factor: 0.9
$Shield Factor: 1.0
$Subsystem Factor: 0.35
$Lifetime: 0.1 ; 0.3
$Energy Consumed: 1.0
$Cargo Size: 0.0
$Homing: NO
; $Homing: YES
; ;; the following indented fields are only required when $Homing is YES
; +Type: HEAT ;; Legal: HEAT, ASPECT
; +Turn Time: 0.15
; +View Cone: 180.0 ;; See 70/2 = 35 degrees from ahead to maintain lock.
$LaunchSnd: 80
$ImpactSnd: 85
$Flags: ( "player allowed" "Flak" "Remote Detonate" "inherit parent target" "spawn PromTorpChild,1")
$Icon: iconPromS
$Anim: PromS
$Impact Explosion: none
; $Impact Explosion: ExpMissileHit1
; $Impact Explosion Radius: 65.0
; $Dinky impact explosion: ExpMissileHit1
; $Dinky impact explosion radius: 5.0
$Spawn Angle: 5
$Name: Prometheus T LR
+Title: XSTR("GTW-5 Prometheus Torpedo", -1)
+Description:
XSTR(
"Hull Damage: 77,14/s
Shield Damage: 85,71/s
Subsys Damage: 30/s
Energy usage: 2,86/s
Range: 15300m", -1)
$end_multi_text
+Tech Title: XSTR("GTW-5 Prometheus T", -1)
+Tech Anim: Tech_GTW-5_Prometheus_S
+Tech Description:
XSTR("", 3277)
$end_multi_text
$Model File: none
@Laser Bitmap: laserglow03
@Laser Glow: 2_laserglow03
@Laser Color: 20, 150, 20
@Laser Color2: 20, 150, 20
@Laser Length: 10.0
@Laser Head Radius: 1.0
@Laser Tail Radius: 1.0
$Mass: 0.2
$Velocity: 750.0
$Fire Wait: 0.35
$Damage: 30
$Arm distance: 400
$Detonation Range: 500
; $Blast Force: 200.0
; $Inner Radius: 20.0 ;; radius at which damage is full
; $Outer Radius: 140.0 ;; max radius for attenuated damage
; $Shockwave Speed: 30.0 ;; velocity of shockwave. 0 for none.
;$Dinky shockwave:
; +Shockwave damage:
; +Shockwave damage type:
; +Blast Force: 100.0
; +Inner Radius: 5
; +Outer Radius: 30
; +Shockwave Speed: 6
; +Shockwave Rotation:
; +Shockwave Model:
; +Shockwave Name:
$Armor Factor: 0.9
$Shield Factor: 1.0
$Subsystem Factor: 0.35
$Lifetime: 1.0 ; 0.3
$Energy Consumed: 1.0
$Cargo Size: 0.0
$Homing: YES
;; the following indented fields are only required when $Homing is YES
+Type: HEAT ;; Legal: HEAT, ASPECT
+Turn Time: 0.15
+View Cone: 180.0 ;; See 70/2 = 35 degrees from ahead to maintain lock.
$LaunchSnd: 80
$ImpactSnd: 85
$Flags: ( "player allowed" "Flak" "Remote Detonate" "inherit parent target" "spawn PromTorpChild LR,1")
$Icon: iconPromS
$Anim: PromS
$Impact Explosion: none
; $Impact Explosion: ExpMissileHit1
; $Impact Explosion Radius: 65.0
; $Dinky impact explosion: ExpMissileHit1
; $Dinky impact explosion radius: 5.0
$Spawn Angle: 155
#End
;=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=
#Secondary Weapons
$Name: PromTorpChild
+Title: XSTR("PromTorpChild", -1)
+Description:
XSTR(
"Hull Damage: 11,25/cu
Shield Damage: 9/cu
Subsys Damage: 9/cu
Range: 1900m
Type: Heat-seeking
Turn Time: 0,85s
View Cone: 100 degrees", -1)
$end_multi_text
+Tech Title: XSTR("PromTorpChild", -1)
+Tech Anim: Tech_GTM_MX-64_Rockeye
+Tech Description:
XSTR(
"", -1)
$end_multi_text
$Model File: none
@Laser Bitmap: laserglow03
@Laser Glow: 2_laserglow03
@Laser Color: 20, 150, 20
@Laser Color2: 20, 150, 20
@Laser Length: 10.0
@Laser Head Radius: 1.0
@Laser Tail Radius: 1.0
$Mass: 0.2
$Velocity: 750.0
$Fire Wait: 0.5
$Damage: 45 ;; damage applied when within inner radius
$Blast Force: 200.0
$Inner Radius: 20.0 ;; radius at which damage is full
$Outer Radius: 40.0 ;; max radius for attenuated damage
$Shockwave Speed: 0.0 ;; velocity of shockwave. 0 for none.
$Armor Factor: 1.0
$Shield Factor: 0.8
$Subsystem Factor: 0.8
$Lifetime: 1.9 ; 1.9
$Energy Consumed: 0.0 ;; Energy used when fired
$Cargo Size: 4.0 ;; Amount of space taken up in weapon cargo
$Homing: YES
;; the following indented fields are only required when $Homing is YES
+Type: HEAT ;; Legal: HEAT, ASPECT
+Turn Time: 0.15
+View Cone: 180.0 ;; See 70/2 = 35 degrees from ahead to maintain lock.
+Seeker Strength: 7
$LaunchSnd: 87 ;; The sound it makes when fired
$ImpactSnd: 88 ;; The sound it makes when it hits something
$FlyBySnd: -1
$Rearm Rate: 4.0 ;; number of missiles/sec that are rearmed
$Flags: ("child" "inherit parent target" "no emp kill") ;;
; that trail was actually a slightly ill-fated try for "curved lasers" which someone requested in the chat - I'll try that again another time
;$Trail: ;; Trail cannot be set if Exhaust is set
; +Start Width: 0.4 ;; Width of trail nearest missile
; +End Width: 0.1 ;; Width of trail before it "evaporates"
; +Start Alpha: 1.0
; +End Alpha: 0.0
; +Max Life: 0.2 ;; how many seconds before trail disappears
; +Bitmap: newmiss3 ;; Bitmap used to draw trail
$Icon: iconMX-64
$Anim: mx64
$Impact Explosion: ExpMissileHit1
$Impact Explosion Radius: 50.0
$Transparent:
+Alpha: 0.85
+Alpha Min: 0.1
+Alpha Cycle: 0.01
$Name: PromTorpChild LR
+Title: XSTR("PromTorpChild", -1)
+Description:
XSTR(
"Hull Damage: 11,25/cu
Shield Damage: 9/cu
Subsys Damage: 9/cu
Range: 1900m
Type: Heat-seeking
Turn Time: 0,85s
View Cone: 100 degrees", -1)
$end_multi_text
+Tech Title: XSTR("PromTorpChild", -1)
+Tech Anim: Tech_GTM_MX-64_Rockeye
+Tech Description:
XSTR(
"", -1)
$end_multi_text
$Model File: none
@Laser Bitmap: laserglow03
@Laser Glow: 2_laserglow03
@Laser Color: 20, 150, 20
@Laser Color2: 20, 150, 20
@Laser Length: 10.0
@Laser Head Radius: 1.0
@Laser Tail Radius: 1.0
$Mass: 0.2
$Velocity: 750.0
$Fire Wait: 0.5
$Damage: 45 ;; damage applied when within inner radius
$Blast Force: 200.0
$Inner Radius: 20.0 ;; radius at which damage is full
$Outer Radius: 40.0 ;; max radius for attenuated damage
$Shockwave Speed: 0.0 ;; velocity of shockwave. 0 for none.
$Armor Factor: 1.0
$Shield Factor: 0.8
$Subsystem Factor: 0.8
$Lifetime: 19.4 ; 1.9
$Energy Consumed: 0.0 ;; Energy used when fired
$Cargo Size: 4.0 ;; Amount of space taken up in weapon cargo
$Homing: YES
;; the following indented fields are only required when $Homing is YES
+Type: HEAT ;; Legal: HEAT, ASPECT
+Turn Time: 0.15
+View Cone: 180.0 ;; See 70/2 = 35 degrees from ahead to maintain lock.
+Seeker Strength: 7
$LaunchSnd: 87 ;; The sound it makes when fired
$ImpactSnd: 88 ;; The sound it makes when it hits something
$FlyBySnd: -1
$Rearm Rate: 4.0 ;; number of missiles/sec that are rearmed
$Flags: ("child" "inherit parent target" "no emp kill") ;;
$Trail: ;; Trail cannot be set if Exhaust is set
+Start Width: 0.4 ;; Width of trail nearest missile
+End Width: 0.1 ;; Width of trail before it "evaporates"
+Start Alpha: 1.0
+End Alpha: 0.0
+Max Life: 0.2 ;; how many seconds before trail disappears
+Bitmap: newmiss3 ;; Bitmap used to draw trail
$Icon: iconMX-64
$Anim: mx64
$Impact Explosion: ExpMissileHit1
$Impact Explosion Radius: 50.0
$Transparent:
+Alpha: 0.85
+Alpha Min: 0.1
+Alpha Cycle: 0.01
#End
Add that to some ship and mission loadouts, and have fun

. (you might want to reduce the secondaries turn time for balancing...

)
Note: The "LR" variant is seriously overpowered - with that range and turn time, you can simply fire into the blue and wait for the shots to search their own targets...
For further balancing, you might also want to increase the turn time of the "normal" PromT - 1.0 is about usual for the better dogfight missiles, but they are not that fast... so maybe give it a turn time of about 0.5 to 2.0, as you see fit

.
Actually, when testing it, I had a bit of a "deja vu"... All those green blobs moving like a school of fish hunting some prey very much reminded me of the end of Stargates' southpole-battle, just this time the drones were greeen...

...I'd have uploaded this to FSMods with a few test-missions (Asteroid Dogfight & Shivan Gauntlet), if uploads were working currently (please repair them soon, achtung

) .
And nope, I tested this in another missiion... as Shivan, of course ;o) . ':.:'
(over 100 GTVA pilots were lost to that weapon in it's final test run - oops...

)
P.S.: I actually found an even more promising use of that concept... But I'll still have to flash that one out a bit.

Also, I haven't yet tried to set the parent to ASPECT... I'd guess that wouldn't get any positive effects, but maybe I'm wrong...
*out*
