Well, after spending an hour pushing on the belt, lashing the belt with rope, clamping the belt, not loosening the pulley nut, reading this thread, finding out I don't own a doorstop, I'd like to add that a coil spring compressor worked like a charm to tension the belt enough to grip the pulley and break loose the nut.

Untensioned
Just threw a rag over the belt to prevent damage. It helps that I have my radiator and fan out, too.