I have and idea that might work, as with bearing races that you can't drive from behind for one reason or another.

Drill a large enough hole in the center of the piston to get a hacksaw or Sawsall in and carefully work your way to the perimeter of the piston, use a hand file or hacksaw blade to do the last few thousands so you don't mar the cylinder walls.

With the piston's integrity thus diminished, it may free up the circumference and release itself??

Good luck, I'd really like to get my nose in there with you as I love a challenge, Joe