For the record: it actually (luckily!) seems that the documentation about simulation variables is (partly) outdated, as far as “write access” to simulation variables is concerned:
https://docs.flightsimulator.com/html/Programming_Tools/SimVars/Aircraft_Simulation_Variables.htm
For instance the variable ‘LIGHT NAV’ (“navigation light on/off state”) is marked as “read-only”, however I was just able to successfully set it with the provided (in the FS2020 SDK) SimWatcher example, at least with the “Extra 330 Asobo” aircraft (I did not yet do extensive tests with other variables or aircrafts).
“Succesful” in the (expected) sense that the corresponding light switch in the cockpit got set (“on” or “off”) and the light was enabled/disabled accordingly.
This is with the current Flight Simulator 1.14.5.0 release.
