This eurorack module connects MIDI over USB with the internal MIDI bus, and will work in both USB device and host modes. It also offers stereo audio streaming into and out from the rack when running in device mode. View this project on CADLAB.io to see renders of schematics and PCBs, as well as current issues.
The code for this module can be found on GitHub
More info on the M2IDI system can be found on the Hackaday project page