I would actually request this NOT be added. Your ship has a pitch ladder within planetary flight, and just seems like one more toggle to worry about, and another keybind. I'm not sure what the application would be to have one other than preventing crashes with the planet surface.
As I mentioned previously - You can inhibit the auto leveller, has been like that since horizons launched.
Personally I would like a way to turn it back on when running with 4PIPS to ENG and full throttle, since the ship no longer automatically maintains a 0 degree pitch angle.
The op's idea for a toggle in the functions panel would be great.