If you can “plug in” to the SPI bus, then you can just wire in an SD card frame and do the rest in software.
Of course, the SPI lines are not conveniently exposed on a stock Arduboy. But once you have those, it’s my understanding that most SD cards can be read with the card directly connected to the SPI bus ([SD Card SPI Mode details]).
This will still require reworking the bootloader, to have it pick stuff out of the SD card and stuff it into flash. And - it might require creating a dead simple SD card memory map so that the bootloader does not have to pick it’s way through the FAT to find the files. And that then means you need custom drivers to create that custom SD card format from the source system (likely either Windows or Linux).
But at the end of the day, once the “game card” is written, you would have something quite small, light, portable, and not needing any additional batteries. It still remains to be seen, though how easy it is to carry around an Arduboy with the extra pins in use for an external device.
As we’ve seen in some of the other discussions, it appears that access to some extra pins will be tricky, to say the least. With the overall width of a standard pin header only being about 2mm, I’m wondering if one edge of the Arduboy could provide a header row for four of the unused pins, straight into the edge.
Dreaming here … a four pin header beside the micro USB port.
Ok, back to reality now!