Quick update. Next proto board is starting to shape up. It makes use of two AVRs. An attiny2313 running AVR-USB firmware to support the USB interface. This is connected to a atmega88 running my DISK ][ RWTS firmware via a serial interface. The USB will start with a CDC (communications device class) type device, though I could potentially change this without changing the hardware.

I probably have another couple of weeks to finish the layout, get the parts I need and build the board