MIDI input is fairly simple, as BASSMIDI is basically just forwarding the received MIDI data to the user's callback function. Similarly simple MIDI output functions could be added, but I imagine most interest would be in playing MIDI files/sequences rather than just sending individual events, ie. playing BASSMIDI streams on MIDI hardware. BASSMIDI streams are tied to an output device, not only for sound output but for timing too. It's quite a big change to send events to MIDI hardware instead. It would be a nice feature to have, and I wouldn't rule out it appearing in future, but it won't be simple!