I think ABS doesn't have pump to generate presurized brake fluid, pressure created from your foot. Mostly Abs just have a release valve(pressure valve) per channel and open it in case of lock in order to reduce pressure on the caliper and the most recent units (EBD) change dynamicly the pressure distribution.
ESP in the other hand has a built in pressure pump to create the pressure to engage the brakes even if the driver doesnt press brake pedal.
For me isnt possible to customize (piggy backing) the abs unit cause it has 3-channels. And to be honest i dont think its is possible, also the electric drive is better cause it has proportional effect and can control the braking torgue better (maybe PID) instead of pulsing on-off.
I found that tapping into the VR sensor doesnt affect ABS. Racelogic aftermarket traction control do this succesfully, no problems reported.
So becoming possible, i just found some chips for VR sensor interfacing (LM1815, MAX9927), actually is just a conditioner circuit.
NCV7001 chip offer 4 VR sensor support
Found this also http://brickems.com/brickrpm/