On the right side of the clutch housing is a screen that you can remove to see clutch, You have to remove the plastic panel down by your feet. I would try engaging the pto with the engine off and have someone hold clutch pedal down and see if you can turn the pto shaft by hand.
diesel466 - Thanks to the tip from jd110, I bypassed the correct interlock and can easily start the tractor with rear PTO engaged. I removed the screen on the right side of the bell housing and can see both clutch disks. With the engine off, the clutch pedal clamped all the way down and the rear PTO engaged, I can't turn the PTO by hand. Both clutch disks appear to be about the same thickness. The main transmission clutch disk has about .030"+ of float, and can be moved back and forth. The PTO clutch has no space on either side and can't be moved.
From looking at the transmission diagram, it appears that the throw-out bearing is likely of the carbon variety, rather than the ball bearing type. If so, it might be the throw-out bearing that's shot. The operation of the 2-stage pressure plates is a mystery to me, but even a worn pressure plate shouldn't cause this problem.
So, I'm back to the premise that either the PTO clutch is frozen, or I can't get enough clutch travel to disengage it, possibly due to a worn throw-out bearing. Either way, it's likely that I'm looking at splitting the tractor in the spring.