To push this forward slowly but slightly more firmly, I will be making the new behaviour (proposed above) the default implementation in ATC-pie, adding a toggle option called "guess FGMS transponder modes" to revert to things as they were (making modes GND and C-vs-S undiscoverable). I will keep it as long as we see fit, but hopefully not forever.
The "dev" branch already implements this. The "xpdr" branch announced above will be discontinued as soon as dev is merged into a stable numbered version.