I suspect the contents of the ROM is not really all that interesting. You can get a pretty good idea of what the code looks like just by viewing the microcontroller as a black box and knowing what the keyboard does. There will be a routine that scans the key matrix continuously, spitting out the appropriate code each time a key is pressed. I would assume that the keystroke buffer is implemented in software in the OS, and auto-repeat of keys likely is as well. Hang a DSO or logic analyzer on the keyboard interface and you can probably work out the details fairly quickly.

I'm pretty sure there is nothing fancy going on, AFAIK the pre-ADB Mac keyboard is a relatively "dumb" device.