Every turn, send large sums of money via paypal to all players that upload on time. I reckon this would work very well, although this idea does suffer from one obvious drawback.
Maybe PBW could track how good ppl are/ aren't at uploading and compile some sort of rating which would be publicly viewable. Everyone would want to maintain a "good" rating and (theoretically) everyone would get more punctual because ppl with bad Ratings would have a hard time being accepted into games. But then again, maybe this idea sucks as well.