ps/2 mouse to 128 mac is dead easy with a mcu (I recently did it for my //C but the pinouts are abit different ) basically there's 2 signals per axis, it works like a normal rotary encoder per axis, and has a quad comparator to "sharpen up the signals" (which you wont need)

If you know how to hook up a ps/2 mouse to a mcu (which is a standard issue example for every mcu ive ever used) , and you know how to use a rotary encoder just combine the 2 and off you go

If you need more details I would be more than happy to provide them, but its late and I did not make any notes, so give me a day or 2 so i can properly whip up some documentation and theory

ADB is ticker because it actually is a communications protocol, seems like some mutant form of I2C but since I have no ADB system, and have never seriously looked at it, I have no further info at this time