There is plenty of room for a larger chip behind the battery tray, you just have to run longer wires to get to the hucard slot. That's how I've done expresses in the past. Heck you saw the pics of people installing 8PDT switches there before!
If we cant get a good tiny chip to work, then we might have to consider going with a larger (and more user friendly / pretty) chip that just mounts behind the battery tray.