Just an idea that may fly or not.
Probably the cleanest way FC can affect 50% heals is to make it a hostile nano that one class executes on another in pvp. It doesnt even have to be 50%, it can be a line of nanos with a whole range of percentages which block healing.
This way all the programming "complication" that comes with trying to determine whether a player is being attacked by a mob or player to determine whether a heal is 50% or not, becomes a moot point. If such and such player has the hostile nano run on him, then all healing nanos cast on that player have their effectiveness reduced, regardless of whether they are being attacked by a mix of mobs and players.
I'd say if such a combat debuff line exists give it to a class that needs help, like Engineers (it could even take the form of some anti healing null space aura) even though I can't think of a plausible RP reason (at the moment) that would explain why Engineers would have such a nano line. Or give it to Crats.
