How does that board fit into the Mac? Without cutting into the case, I mean.he has a solution for that too
http://www.synack.net/~bbraun/mackbd/index.html
use standard ps/2 mouse / keyboard
with 128k/512k/plus annnnnnnnnnnnnnd now supports adb as well.
With a relay in the right spot, one can plug in the device and the board would automatically set it for which set up it is without needing a jumper.
BTW - I was converting old serial mice to Mac back in the 1990s, so that wiring scheme works great if one had an adaptor!