Do you know if there are any products like that on the market now, which has a SPST switch on Toe Down that pulls the Tip to ground?
I don’t know how they’re configured, but I know of at least two options for expression pedals with momentary switch:
Mission Engineering’s Aero Line has an upgrade kit for a momentary Switch:
Headrush’s standard EXP for their Looperboard has a momentary switch:
(This one seems to have a separate TS switch output jack that get shortened.)
Or are you planning to mod your expression pedal to use this feature?
I would mod my pedal. I have two EP-1, one genuine and one that was a more than 15 year old was pedal originating from Poland, which has the exact same casted pedal housing as Mission Engineering, down to the last fraction of a hair identical. I threw away the innards and retrofitted a quality pod.
If you need a tester, I can mod either one of my pedals to your specifications any time.