A guy on Polish FB Helix Group claims that he used toe switch expression pedal with his HXFX and it worked fine - he could switch between volume and wah. The update version was above 3.15 though. After so mixed info on the web and so few confirmed I tend to lean towards stand alone pedal with volume and wah before preamp though.
Maybe somebody form Line6 supprt could give me a clue please. ;)