Doh! I take it back: just after reloading SimWatcher and only loading (and trying to set) the LIGHT NAV value it did not work anymore.
After fiddling around with other “LIGHT” related variables (and also loading e.g. the SimVarTokens.simvars set) all of a sudden I was “able” to toggle the “Strobe Light” switch (in the “Extra” aircraft). However I was in fact trying to set other light variables (e.g. LIGHT PANEL ON).
And I tried with both LIGHT NAV and LIGHT NAV ON variables.
So there seems to be something “messed up” in FS 2020 with regards to light simulation variables still, and hence the “read-only” state in the documentation.
I will do further experiments (possibly the example SimvarWatcher application does something weird here), but it seems we are still out of luck (to set light related variables via “setters”).
UPDATE: In fact it seems that setting a “LIGHT *” related variable simply controls the “last succesfully set variable”: for instance I had once case where the LIGHT NAV variable (setting it to either 0 or 1) actually set the engine throttle lever position (to 0 and 100% respectively): and I had set GENERAL ENG THROTTLE LEVER POSITION:1 just before (with SimvarWatcher).
Sometimes setting a LIGHT variable also influences the last "manually toggled light switch (with the mouse, in the cockpit), but not always (hence my initial belief above that LIGHT NAV would work, as I had toggled this myself manually in the cockpit).
In short: “does not work yet”