You're in luck, strobe! The Wacom protocols
are documented in the Developer section of Wacom's web site. Not only that, but...
I have fully implemented the Wacom UD protocol in a REALbasic application that reads a
serial Wacom tablet, dynamically displays the state in a window, and allows you to send commands to the tablet to change its mapping, etc. It even lets you draw lines in a little graphics control.
If you would like a copy of this application and believe that you can write an HID driver I would be most interested in assisting you. I believe some hooks to the IOKit will be required to communicate with the (POSIX) serial port, meaning it would likely have to be implemented in the limited subset of C++ required for IOKit drivers.
From my investigations I've determined that the most promising course for making serial and ADB UD tablets work on Mac OS X is to make the Wacom tablet appear to the OS as if it was a newer USB Wacom tablet, and then the Wacom drivers would happily do the high-level interpretation. (The UD protocol has only been extended, not altered.) So all we'd really need to implement is a serial-USB bridge.
Are you familiar with IOKit? Do you have the skillz required to pull this off? Folks like me would be most grateful if this could be made to work! I've contacted Wacom about this but they have no interest in "supporting" older tablets. A developer I spoke to at Keyspan seemed to think the idea of a serial-USB bridge was fairly promising. I just haven't had the time or inclination to get down with IOKit since I've been so busy with SDL / OpenGL.
adambyte: Is your tablet being detected in Mac OS X without having the Wacom driver installed, or do you mean it (almost) works under 9?