I finally got a DB-9 cable, and I can report that PS/2 mouse emulation works great! That is, after I finally found my PS/2 mouse buried in a box of unrelated stuff, and stopped trying to plug it into the ADB port on the board.

If it matters, this is a Logitech 2-button roller ball mouse. Using it with my Mac Plus is a pleasure. The real Mac Plus mouse feels like a lead brick in comparison.

While clicking around the Finder and reading the "about" box, I was reminded that the Mac I'm typing this on now has 2000x more RAM than the Mac Plus. Good times.

Once again, awesome stuff!