Hello elsp1991,
Using a window motor as a servo is a great solution to this system. In the mean time, I had the idea of piggy backing the ABS system it self. Instead of a linear actuator, or motor controlling the brake, why not use the ABS pump it self. Potentially the ABS pump has valves, that close off the break pedal to individual brake lines, and then applies brake line pressure using the ABS pump (you need a 4 channel/valved pump).
Great attention to detail with the VR sensor. I never thought about that, however, I do agree that tapping into the VR signal could create extra draw that would alter the signal into the ABS system. At the moment, the only thing I could think to do, would be to mount an additional sensors. The traction control system I intend to use was only for off road use, and would not be using, and/or disabling the ABS during use. Here is a notion, the brake pedal signal could disable the traction control system during use, thus working independently from the traction control (set relays/logic gates to toggle signal feed into TCS). With that notion, I do not believe the ABS actually works until you hit the pedal, because it does not randomly brake while driving (excluding any vehicle with on board TCS traction control system).
Back to linear or window motors mounted to a floating axle. The unsprung weight of the floating axle causes a lot of G-force on what ever is mounted to it. I would be cautious to sensitive, or precious electronics being mounted to this type of environment (don't forget about water).
Also, I'm pretty stoked that at least one other person is interested in this topic.