As far as I am aware, a Hall Sensor can be used instead of a simple switch to operate the regenerative braking and power cutoff feature.
GM Canada actually supply
hydraulic brake kits with integrated Hall Sensors and magnets which are supplied pre-wired to plug directly into the MP4/5, SP4/5 or MP Edge brake connectors:
When I first discovered the +5V feed in the brake connector wiring I initially hoped this would allow variable regen to be used, but I'm pretty sure that they are digital Hall Sensors simply function as a solid state contactless switch
(either fully On or fully Off).
Having unsuccessfully used a potentiometer to try and control the level of regenerative braking by varying the brake signal output voltage, I am pretty sure that even an analogue Hall Sensor would still behave the same as a simple On/Off switch when used with the Magic Pie controller, as there is no provision within the controller to vary the braking effect based on the signal voltage.
While the output signal voltage remains above the trigger voltage, the braking function will remain turned Off, but as soon as the signal voltage drops below the trigger voltage, the braking function would instantly be switched fully On.
Alan