Okay, so, you know how I said that using an ATMega8515 would be too easy?  Well, I proved it ;)   Still fits on a 50x50mm board (if you do it diagonally as I did… Seeed Studio is going to hate me for this one).  The majority of the parts are on the underside of the ATMega.  This picture isn’t the very final one — I’ve added a (non-standard) ICSP port to it as well.

This has some benefits (smaller case, nice mitered edges everywhere, the knowledge that it can meet timing to work with 6-button pads, etc.)  I plan to order both, and write the code for both designs and post it.  Updates to follow when the boards arrive and are tested.