well, got a chance to look at this again and make some progress, well - sort of, I think.
spent a little time whacking the top of the piston with a wood dowel and hammer. it seemed to me that the piston did move down, maybe half the width of a piston ring - viewing through the exhaust port - the bottom ring is right at the top of the port, so - could kind of gauge that more of it was visible and had moved down.
so, I put the head back on to rock the flywheel, and it sort of pops back to where it was, in one sudden movement as soon as I started to rock the flywheel with a socket on the flywheel nut. continued rocking of the flywheel did not appear to create any movement, rocked it for a few more minutes, put oil back in the cylinder and walked away.
so, is it just stuck? or is it likely a ring is busted and stuck in either the transfer or intake port? I don't know, it was just odd how it sprang back like that, but good news is it definitely moved, so - if it is just stuck, any oxidation, dried up old oil, or bond between the rings and cylinder should now be broken, so the penetrating oil should be able to work in there better now that the piston did actually move some.