And wasn't it nitro2k01 who added this patch to mGB (can't find the coresponding post here)? AFAIK, Arduinoboy code is not involved, hence it should work with the nanoloop adaptor, too.
I simply added an offset to the MIDI address in the mGB source code. It turned out that the versions I created crashed when receiving data on incorrect channels. This was, I think because for data received on incorrect channels, only the first byte would be read, and the next 1-2 bytes would be interpreted as the start of a new message. But in theory, this should be possible.
The recommended way as of now to achieve this is within Arduinoboy.