Virtual, variable resistors, with saved presets recallable by MIDI

Nope - but it positions everything round the centre-point of the audio electronics supply , so should be OK.

If one could identify the internal '0v' , you could use a couple of 2.5v references (TL431's?) round that to get the arduino's 5v - that would be a better approach.

But I think it'll work.

Allan