Yep, I'd go with George's idea. Lucky that pistons are mid stroke, as that gives teh best angle of attack for your hammer on top of the pistons. I had a 2002 (BMW style) that was hopelessly seized. I removed the head and just got western on the pistons with a big hammer. Got 'em freed up, rough honed the cylinders with the pistons in place. Put it all back together, and damned if the car didn't run!

I'd think if you don't care about the block... you should be able to get them things to move even if by brute force.

Another option perhaps? Cut the pistons around the wrist pins. Oxy/acetylene would make quick work of that aluminum. Maybe even a hole saw, if you have one that gets you deep enough.

It's probably only one piston that's stuck, so.. I'd do the rod caps first and find out which ones move, which ones don't.

--
-Matt I ♥ my ♂