I'd go double-barrel TMI on them, frankly. "Those cheese enchiladas with extra sour cream look gorgeous, Claire, but if I eat even half of one I'll spend the rest of the night in the bathroom, and you'll all be 'sitting in my pew' at service tomorrow." Make it abundantly clear that they are asking you to make yourself - and a significant number of the people around you - miserable because they can't be bothered.

I probably wouldn't be above passive-aggressively bringing pretty tent cards and handing them to people as they put their contributions on the potluck table, either. (That might get more traction if you talk to the other folks with dietary concerns and get all of THEM to fill out ingredient cards first - peer pressure for the win, or something.)