@nicnut Found something.....
You can change midi driver with the message to Pd [midi-setapi x (..... where x is the index of the driver in the Media Settings list (ALSA-MIDI, OSS-MIDI or whatever) starting at 0 if there is actually a choice, ((or requiring 1 if there is not...???... but then we would not be using it)).
Unfortunately this opens the Midi setup menu.
You can then re-apply the Midi settings for that driver with a [midi-dialog 0 0 0 0 1 0 0 0 1 2( message....... with the correct data of course as collected from Pd like this......... get_set_midi.pd
It will be applied even though the Midi setup window will be left open showing the previous setup (which could be the same of course).
A bit of a messy explanation I'm afraid. I don't know how to close that window, but it shouldn't matter.
So something like......
[midi-setapi 1, midi-dialog 0 0 0 0 1 0 0 0 1 2(
sent to Pd.