I have seen v-belts crossed to change rotation. Can you just put a pulley on the PTO, 2 idlers lower down to get the belt going forward, then have the belt cross on it's way to the deck? Takes a pretty long belt, and the belt is rubbing on itself, which would seem like it would have to reduce belt life somewhat.

Ever seen pix of an old belt-drive threshing machine with the drive belt crossed? Same principle.

Edit-I re-read your post, you can also run a longer belt around a couple idler pulleys, so the load pulley runs on the outside of the belt, if your belt doesn't have a long enough span to cross it.

OR(smart-ass alert!)you could use 2 wheelbarrow tires, one on the PTO, and the other on the mower driveshaft, running tightly against each other.