no expert on modding, don't know if it can be done, but i had this idea :-

something like the novalith cannon for stars!! (for multi-star games)

not suggesting it wipe out all planets or anything, but a drop to overall economics around that star would be reasonable, maybe changing planet types to something else, i.e. terran - desert.
 possibly it would even change/remove extranious phase lanes(all planets would still be connected to the star system, but 'loops' would end up as 'strings' if you get my meaning) this would be random though, so while it drops the enemy economy it may be a damn sight easier to defend for them(or maybe not )

only good for BIG games obviously, it should be almost too expensive though, maybe set your own economy back(less than the enemies after its used) to build it/ destroys the planet its built in when fired