As far as I know, nothing…no aux, no midi, can control the output volume on the fly.
Which, I agree, WOULD be awesome.
You can control the other parameters, though….so I fake it sometimes.
Like, in PitchFuzz…the gain knob can add some loudness.
In a lot of the reverbs/delays…you can start pretty wet, with a lot of output volume…and switch back the mix value to dry-er…which gives you some apparent volume.
You can add an octave in, or an additional delays/voices… Etc etc.
Those are what I do.
Usually with the expression pedal, not an aux switch, though. So much easier, and you can dial it back to get the right stage volume.