Another thing I recall being mentioned hereabouts (possibly from trag?) is that the Plus (& earlier) mouse port take the signals direct from the encoders on the mouse ball: ie, that intead of a chip inside the mouse, the decoding is done inside the Mac. Hence, you can take a newer ball mouse (PS/2, serial, ADB), tap those lines, and feed them into a Plus.

This is just off the top of my head, so your own research is advised.