I’ve accidentally used a footswitch of the incorrect polarity before, and the behavior was exactly what you seem to want, the action didn’t happen until the release of the pedal. You say you tried this before, but on what function?

Can you elaborate?

To be honest a pair of tablet style footswitches strapped together so pressing one would press the other sounds like it would work if wired to the FC7 input’s Perf. Up/Down functions. It’s not a binary function (unlike sustain with an on/off state) so pressing the contact immediately triggers the function and releasing it does nothing. I don’t see how not releasing the on state for a long time (which would be the reversed polarity switch’s normal condition) would interfere with the reception of the on state from the normal polarity switch… they are two different functions, not the on/off state of a single function.

Before you go your far more elaborate and complicated route, I’d give this potential solution another try.