Update:

My modded player developed stability issues -- random crashes and the like.

My theory is mechanical issues -- the memory expansion card adds a lot of stiffness to the main board, and when the main board is subsequently screwed back into the stainless steel enclosure, it wants to bend to match the enclosure screw posts (flat?). This puts stress on the connections between the two boards, and I think something just "gave" in the end.

So, I've put enough hours into this for now, and tonight I disassembled the player and removed the memory card again. Not quite as clean a removal this time around -- destroyed several of the test pads, but they're not really important or needed for anything other than the (removed) memory card, so no big deal.

Back to 16MB for now, but I do still have a couple of pairs of chips to do the stacking method with eventually.

Cheers