Update: I found a data sheet
for one of these things. It completely takes over the ACK and BUSY lines (controllers 1 and 2) to do data transfer without talking to the host. It does pass back the PAPER-END, SELECT, and ERROR lines (controllers 3-5) as usb packets, but only once per frame. The driver relies on being able to test these once per clock cycle.
Re-wiring the cable to skip controller lines 1 and 2 and re-writing the driver to be USB aware so that it could force one-byte frames, this might
be workable. It would be something like 1000 times slower, but that would probably be ok because it only has to be as fast as a human can push buttons.
As wired and as written, this setup would not work at all. =(