For the HX Stomp, I see that there is the following setting to auto-engage the wah block with an expression pedal:
Bypass assign – Switch Types – FS1, FS2, FS3, EXP1, EXP2
You can then set the percentage of travel before engaging the wah as well at the delay time to engage.
I use an MAudio EX-P Expression Pedal that is plugged in to the EXP slot of a Tech 21 Midi Moose. The Expression pedal is controlling the wah on Midi CC 80.
You can see my dilemma... Switch Types – FS1, FS2,