It's an interesting idea, however in practical terms I wonder what this actually does to make a plasma carronade a better weapon than it already is? Presently the plasma carronade essentially works like this: if your guns are in range of the target, you win. The check on it is very short range that cannot be improved with technology (although this does make them a very cheap weapon type to research). So realistically, what does making them even deadlier at that short range actually do to make them a more viable or interesting weapon? At the end of the day, if your guns are in range of the target, you still win.
Comparing to the other special mounts, I think there's a clear requirement that a special mount gives its weapon(s) an additional capability that supplements its existing role rather than merely enhancing it. Laser spinal mounts for example provide a supplemental long-range and alpha strike capability to the reliable DPS of standard lasers; turrets for lasers/Gauss/mesons give them point defense capability (in the case of mesons admittedly this is a VB6 holdover); the particle lance turns a decent mid-range sniper weapon into a surgical annihilation device with its unique damage profile; and in 1.13 railguns will gain a reduced shot capability making them a strong fighter weapon in addition to their existing all-around capabilities.
So for a unique plasma mounting option we need something that will give plasma cannons a new dimension or capability, not just more dakka. Given the existing limitations of plasma, this is a difficult order to fill (R.I.P. plasma torpedoes), but a necessary one.
(Incidentally, HP microwaves also suffer from this problem, although I've suggested the idea of mounting HPMs into a missile to produce an EMP bomb elsewhere. That said, their utility is so narrow and specific that it's hard to conceive of a meaningful alternative mounting for them as a beam gun weapon.)