Wait a minute...you got the "particle spew" flag to work on secondaries? I tried that once, and FS promptly cursed me in machine lingo.
You could have a player-controllable beam cannon, too, with a little creativity with the SEXPs. Something like: (and forgive me if I didn't write the code in the proper unparsed format)
$Event
(when
(key-pressed X[or some unmapped key])
(fire-beam
playership
beamturret
sometarget)
)
Then give it some insane repeat count, and a 15 second delay.
Unfortunately, you'd need a separate key and separate SEXP for each target you'd be able to fire the beam at. Shouldn't be too big a hassle for the experienced FREDers out there.