Since USB controllers appear to be an inexorable feature of the future, and an increasing number of machines lack RS-232 serial ports, implement a driver for the USB 2.0 debug port. Apparently there is only one bridge which implements this standard, a device called the NET20DC (if the purchase price would be an issue for you, please not this in your application and we will look into sponsorship).

Please note that while this project should not require large amounts of code, it will definitely be a grimy driver-debugging experience. Warm-up work would probably include setting up two Plan 9 machines to do RS-232 kernel debugging.