That is somewhat correct (or else I'm the one with the misunderstanding now) - the guard cover itself sends no signal, but it physically locks the switch below it in the down position. That switch is momentary, returns to centre, and only signals on up/down.
Ideally, I would like the whole unit to behave as such:
Switch down (i.e. physical cover down) sends [GMC Cover DOWN] and [GMC Switch OFF] at the same time, so that flipping the physical cover down returns the in-game master arm to off and safe. This I don't know how to do.
Switch middle (i.e. physical cover up) sends [GMC Cover UP], so that flipping the physical cover up unsafes the in-game master arm. This I can do with BTN*_OFF, thanks to your advice.
Switch up (i.e. momentary) sends [GMC Switch GMC], so that just flipping it up once arms weapons. This is trivial, I know how to do this.
Failing that, anything to make the momentary-up toggle [GMC Switch ON/OFF] will do, because I already know how to bind the close-cover command.