Thanks! I hope it works for you.

I've made a smaller version of the board that will hopefully fit on top of the discovery board instead of on the bottom, like bigmessowires suggested. For now, I've just swapped the rj11 connector to be for the handset. Perhaps the MAX4619 can find a place on the bottom of the board, but in the smaller arrangement, there isn't as much extra space, since all the connectors are through hole.

For posterity, the section of Guid to the Macintosh Family Hardware (2e) that I'm looking at is Chapter 7, p283:

If a key transition occurs on the Macintosh Plus numeric keypad for the plus sign (+), asterisk (*), or slash (/) keys -- which are uppercase keys on the separate keypad -- the Macintosh Plus keyboard responds to an Inquiry command by sending back the Shift key-down transition response ($71), followed by the Keypad response ($79), followed by the code shown in Figure 7-6.

Click to expand...

The figure is just the list of key codes.Implementing this verbatim results in the computer thinking the shift key is still down after releasing one of the above keypad keys. So, the proper solution AFAICT is to keep track of the PS2 keyboard's shift key, and if it is held down, just send the keypad code, followed by the key code. If it is not held down, send shift down, keypad code, key code, shift up. I'm not a huge fan of trying to keep additional state since it's just something else that could get out of sync, like the caps lock key, but that seems to be the thing to do here.

Also, dougg3 tested and had some problems with his PS2 mouse. The keyboard seemed to work fine for him. I've only tested with MS Intellimice, so I'm curious if there's a protocol problem. This friday I'm hoping to go get a hand full of different surplussed PS2 mice to test against.