Since this has floated back up into visibility I want to point out that what is being proposed is tampering with Safety Of Life systems. If you get it wrong, someone is SOL. The intended use is off road at low speed. Under those conditions it is probably acceptable if not safe to experiment with these mods and I have hope and need for that application.
But if this is being done to a vehicle capable of being driven on the road, one that shares the road with my family, I want you to take precautions against accidental engagement at highway speeds. This includes precautions that render the system safe and/or issue a driver warning in case of hardware failure. Software alone isn't enough.
Even just a partial sustained application at highway speeds can cause unnoticed brake overheating and loss of service brakes. Unlike linked cable parking brake systems, this can apply just one parking brake or the other. That means only 1 or the other could fail engaged. Now when a driver tries to use his service brakes, one engages, the other doesn't causing unintended steering torque. You see how this could go bad? This thread has talked about applying E brake. Your service brakes are the emergency brakes. Cable operated brakes are parking brakes, intended to be applied only when the vehicle is stopped to prevent rolling an unoccupied vehicle.
Even if the OP does take reasonable precautions, posted code that does not include those precautions could be used by others in an unsafe way.
I am not trying to stifle development. I could benefit from this project. I just know that a little knowledge can be a dangerous thing, so I'm trying to give you a little more knowledge to make it less dangerous.