So I decided to build UMDK; a Genesis / Megadrive hardware development cartridge. The project was released as open-source under the; the code and VHDL is all LGPLv3, and the PCB is CERN OHLv1.2. Thus far, I've only soldered one board and on first tests connecting it to the PC has triggered Windows to detect the device and attempt to install a driver (a good sign I presume). Stay tuned for more updates on UMDK - if I can get this working reliably and within a reasonable amount of labour I may sell a few...