Thinking the same as the others. Maybe take the side cover off the tiller and look in there and see if the chain or gear has bound everything up. Sometimes can use a 2x4 in put it in the tines and rock them a bit to see if you can get a wee bit of movement so you can move release the PTO shaft.
Best idea yet!
Part of the problem is: on these the PTO is a direct mechanical connection to the rear tires while the PTO lever is in gear. So as you suggested the tractor has to roll backward a little to unload the PTO tension. Your idea or the 2x4 in the tines, while someone tugs on the PTO shift lever, are likely the most practical next step.