After reading about this project, I decided to revive my ATmega based cartridge emulator. Mine has fewer parts and is way faster, so it should be worth bringing back to life.
But, unfortunately, no pulse. I have no idea where I went wrong, so I said "fuck it" and started to re-implement it from scratch. The previous version was written a long time ago, and was really just a mayhem of code anyway. I'm now trying to tidy things up, and advance to bluetooth instead of RS232 for uploading a new images.
I also used the MCU's RAM last time, rendering it useless for images larger than 4 kilobytes. The idea is to use some of the flash memory, which it has plenty of - with the possibility of ROM switching. I'll keep you posted on the progress.