I think it already falls within powergaming, but as per enforcement it does not appear to be so, thus this suggestion.
First of all - Coding Difficulty:
No coding needed at all, just a rule change/clarification
Scope:
Small, banning shrooms in maints is not a huge project.
Balance:
Shrooms in maints is a big negative for a lot of antags, most of all the sling. For slings the shrooms ruin games, for cults it doesnt help (as they have NV), for terrors it lights where they dont want it, for tators it makes maints less safe than they already are with patrols. Furthermore it is not hard for botany to make them, and they spread so fast that it is not achiveable to get rid of them. The upside of them? Makes botany somewhat more relevant, but botany can already do a lot of memey things, Sec can get NV huds, as can almost all others easily, and isnt maints meant to be dark anyhow?
Furthermore, is it not so that making shrooms in maints is like wrenching pipes with terrors? It is MEANT to fuck over antags, not to make things look nice or such. For example, one will seldom see it unless slings are called. I for one consider the shrooms just as bad as making cams all over maints, wrenching pipes to sabotage terrors, or removing pipes "in case" an AI is malf so it cannot plasmaflood.