I would like to know about this as well. To my knowledge I have not seen a commercial MIDI plugin that can route directly to and from MIDI ports/drivers, but I do remember that the two MIDI plugins here called "midiIn" and "midiOut" attempt to do this:
https://github.com/sleiner/pizmidi/tree/main/pizjuce
Some of the plugins are buggy, so I do not know if they work or not.
I think some of the challenge on Windows is that host apps can gain exclusive control over a port, and I know DAWs like Cubase/Nuendo are aggressive in this regard, but there are utilities to multiply port connections (such as MIDI-OX, Virtual Cable or Bidule), and therefore making it possible for multiple hosts to access the same MIDI input/output.
I would love it if you found a way to do this in a plugin. On an app I believe you should just be able to have access to the output ports from any of the MIDI devices as long as it is not in use by another app (I think it is first-come-first-serve in that regard).
