I've made a quick mission with the "primitive radar" and it works.
All you need is a Fighter with more then one Secondary bank, and the following Events in the Mission.
Not the most elegant approach, but it works for now.
In the first secondary bank you can have the Missile A ( HEAT flag ) and it needs target lock, in the second bank you can have Missile B ( ASPECT flag ), and in the third you have A again, but once the third is selected, the event triggers and reduces the Radar range to the in the event entered value ( the 150 is only a example i used, you might want to reduce the range further).
You can fire the Missiles and they will head for any target in sight.
After switching to a other secondary bank, the radar range will be restored to maximum.
For a better solution you better ask karajorma ( or anyone else for that matter ) since his Expert-Fu beats my Noob-jitsu, when it comes to FREDing.

Anyway, here we go:
$Formula: ( when
( is-secondary-selected "Blue 1" 3 )
( hud-set-max-targeting-range 150 )
)
+Name: Set Primitive
+Repeat Count: 0
+Interval: 1
+Team: 0
$Formula: ( when
( is-secondary-selected "Blue 1" 1 )
( hud-set-max-targeting-range 0 )
)
+Name: Unset Primitive
+Repeat Count: 0
+Interval: 1
+Team: 0
Enjoy!