[robespierre]

So the problem is that the PS/2 mouse protocol only provides for a mouse with X-Y movement and three buttons. That isn't really adequate to support a graphic tablet, with absolute positioning.
So there were no significant graphic tablets using the PS/2 interface. Aside from ADB for Macintosh, the interface used was RS-232 serial, sometimes with a keyboard port pass-through for power.
The IRIX tablet drivers expect to use the serial port.

One way to use a Wacom ADB tablet is a translator board called the Waxbee. It is designed around a USB host, but it should be simple to modify for RS-232.