After much dicking around, seems like the MINI isn't recognizing any signal from the opti board, with or without resistors. It's supposed to switch over automatically when it senses a signal (killing the trac input) but it doesn't seem to be doing that.
In touch with Andy for further advice, but I'm suspecting the MINI has issues.

Why?
1. Opti board is known good using a mouse hack.
2. Tried both with and without the resistors using the MINI - no difference.
As a side note, the ground wire for the MINI's spinner interface does not check out with the other MINI grounds for continuity. I suspect, however, that's supposed to be the case and that it only "hooks up" when the software switch senses the spinner being used, then it kills the ground for the trac.

EDIT: Just tried hooking it up to an XP laptop in case it was a 98se thing - same results. All works fine, installs two Human Interface Devices and one mouse. Just won't switch between trac and spinner inputs when it should.