I recently tried connecting a XBOX 360 Rock Band 3 Clavier / Keyboard controller using its MIDI output to a Katana. I was interested in both seeing current generation hardware working with the Dreamcast without special adapters / converters and to see what the true functional limitations of the Dev.Box's MIDI ports are. The official MIDI adapter for the retail Dreamcast is connected via the Dreamcast's serial port, however the Dev.Box's appears to be directly connected to the AICA processor.
The one game compatible with the retail MIDI adapter is OTOIRE (a music creation software). I ran it in the dev kit and played a few notes on the keyboard. What I was playing was coming out of the speakers and I could use the keyboard's buttons to change patterns / octaves and pitch bend, etc, however I quickly realized it wasn't actually the game producing the sound, but the dev kit itself. To prove this theory, I reset the dev kit back to the system menu and was able to still play music through the Dev.Box.
Expanding on this, I remember seeing that the retail Dreamcast has two pins (49 & 50) on the G2 bus specifically for MIDI out / in respectively. I hard-wired the MIDI cable to those G2 lines hoping to get the same result as the Dev.Box unit, however there appears to be a problem. When I plug the keyboard into the Dev.Box, the keyboard knows, even without having the dev kit on, that a MIDI cable is connected and it switches itself into MIDI mode. But when connected to the retail Dreamcast, it only goes into MIDI mode when I remove the ground and just leave the current sink and current source connected.
Perhaps the Dev.Box has a logic inverter inline with it's MIDI lines and the retail unit requires one? Or maybe there are some pull-down resistors missing and the keyboard's signals are floating?