Shock Ability

I wonder, actually, if you could repurpose the unused Summon command (with the appropriate targeting fixes) to accomplish this. IIRC, it deducts the appropriate MP for your equipped esper, doesn't it? I don't remember offhand if the current coding disables the command at low MP, but the functionality to disable commands in certain circumstances exists (no appropriate weapon equipped, Imp status, etc), and could definitely be adapted.

If you want Shock to cost MP, but not Megahit (i.e. it costs MP when Leo uses it, but not when an enemy does), there's some additional coding you'd need to do, because you'd need to hardcode the MP cost into the function rather than pulling it from spell data, but that's not actually all that hard, either.