Even in parody works, you're inherently walking on eggshells. It needs to be a pretty direct and obvious ciritique of the original material. Something like PETA's Pokemon Red, White and Blue is unchallenged because its so unsubtle they'd have a reasonable defense. It would need to be a lot more than just a line in the mod description to give him a good defence. Even if it was as direct in it's criticism as PETA's game, just because he's legally probably covered, also doesn't mean he can't be taken to court, and unlike PETA, a mod-maker won't have a fortune to burn on an expensive fair-use lawsuit, nonetheless if its a riskier or more complicated case.