Wait, hang on. I'm trying to get that set up now, and running into another issue: UHF isn't working, while VHF is. Weird! For instance, I've set my ELEMENT PTT to VHF, which is "Winwing WINWING Orion Throttle Base II + TGRIP-F16 Btn 24". When I push it, I can see the little white controller icon on the ELEMENT group turn green. I did exactly the same thing to set GROUND to UHF, which is Btn 26. When I push it, the white controller icon on GROUND stays white, and when I say "Ground, pin" the log indeed reports that it recognised the command and inhibited it due to PTT.
Weird. It did work when I changed GROUND to my C keyboard key, just to test, but not that other HOTAS key. I know the button itself works - after all, FoxVox itself picked it up when setting the PTT key!