I managed to do this with expression pedals connected to my RJM MMGT22, and there is indeed an issue with this that needs sorting out. I dont know if the issue is in the Axe3 or the MMGT, but when going from heel down and tuner engaged (tuner NOT muting the Axe3) to toe down, the volume jumps from completely silent to quite high, before starting to track the pedal.
@FractalAudio , any idea if the issue could be with the Axe or the MMGT?