I can't find a button/key assigment allowing changing transponder modes, for example in steam gauges based C172 (off, test, stby, alt). Enabling alt mode when in the air is quite important, the lack of the key is disrupting my real world procedure (I fly using a physical panel, mouseless). I can't even find such assignments in FSUIPC.

It's possible with FSUIPC:

https://forum.simflight.com/topic/90446-transponder-offsbyonalttst/