Brainbread, I'm pretty sure that isn't possible. Weapon damage types are hardcoded, only on-hit effects let you mess with them.
I'm loathe to design a whole heap of large weapons, because right now there are exactly two Blackrock ships that can mount them. I see your point, though.
So I've been playing a lot of USC lately, with the "hostile to all, omnifactory only" setting, and I'm pretty happy with this faction's balance overall at this point. Are there any ships that feel too powerful or too weak? Having killed everything up to a Nevermore with frig escort using nothing but 2 lashers, a few hounds and a few Echo-class scouts (ifed), the designed weaknesses seem pretty okay to exploit for the player right now.
I'm thinking I'll probably make a phase ship next. I also want to have a throwaway frigate for the faction, they're so much fun... a 3fp or 4fp frig without a shield generator, but fast as *** and with decent weaponry. Also considering to make a Nevermore version that has an enormous plasma emitter as the built-in version and functions as a subcapital, but I'm gonna be cautious about making a ton of nigh-identical ships.