Had the same idea, thought to try it on cr-10 aluminium bed... found that the aluminium bed, although it looks like bare metal, is treated and it is not conductive . Next step was to cover the glass sheet in aluminium adhesive tape(for testing purposes), modify marlin to invert the endstop type to be normally open (default is normally closed), connected a wire of the endstop to bed and another tby driver_x - General