I've seen it done with 700c rims. Someone took a very nice Stumpjumper and made a very bad road bike out of it. The brakes almost fit, he used cheap stem shifters that almost worked, and he almost didn't hit his shoe on tight turns.

Thought about doing the same conversion with 650b rims on a disc brake equipped bike. Wasn't cost effective. Would have been better off just buying a hybrid.