Interesting thought but I think the brake switch might only operate the brake lights. Trouble with my particular model it's got too many complications for what should be a workhorse.
From what I've gleaned so far the abs valve block contains two microswitches in series which operate when fluid pressure is applied. The two n.o. switches have internal shunt resistors so no pressure yields 4k ohms, one or both switches operating yields 2 or 1 k and this is connected to a unit hidden within the car that also reads wheel rotation speed and calculates which brake cylinder to preferentially apply fluid pressure to.
The reference earth I'm guessing relates to the state of the valve block pressure switches but how it measures seems to be the crux of the matter.