I'm working on a mono-synth based project for the NES. The MIDI adapter will plug into the controller port (no cable coming out of the cartridge in the front of the NES), and the cartridge will be a simple EEPROM without any specially processors or anything. It will be open-source as well.
This is only a small part of my larger Kickstarter project which you can check-out here if you're interested:
http://www.kickstarter.com/projects/chi … technology