Possible to lock wah into a fixed stop location across all rigs in a performance?

Yes, of course. Just hit the Lock button once you have it where you want it.

Once you lock it in position, the Wah will override any other stomps that were previously assigned into Slot A

If you have Morph to Wah enabled in system settings, this will work if your Wah is set to Bypass at Stop, or Bypass at Heel. (Changing rigs defaults the morph pedal position to Heel).