Well you could use the mGB mods that change the midi channels, so you could have up to three. Though I'm imagining a giant wall of gameboy drones which would be amazing.
Yea but the source midi would still be the fixed channels. Arduinoboy can only send up to max 4 channels, no matter what downstream splitter you have.
If you made an active splitter that could transpose the channels, like the midi solutions devices do, then sure you can control more than four devices BUT STILL ONLY FOUR CHANNELS OF UNIQUE NOTES at a time.
should work with nanoloop usb midi adapter though.. i have a modded dmg-07, can test that combo as long as someone reminds me later