That is an EVO valve. It regulates the bypass pressure based on vehicle speed and handwheel rate ( full boost for evasive manuvering). I have looked into making a controller for an EVO valve and it would be pretty easy for the vehicle speed but a little more difficult to package the steering wheel rate sensor.
My feeling is that it could be done with a 2 axis accelerometer.
As we spoke here :
http://www.vettemod.com/forum/showthread.php?t=8693
The steering sensor problem remains.
Accelerometer might be a problem soo many vibration to screw the acceleration integration. A magnet on one wheel with an hal sensor would be more reliable.
....
That make me think about a post on hackaday where a guy was using a laser mouse to sense the speed of a robot by shining at the carpet below it.
You could sense the steering column, provided you can keep it out of the dust with a casing and give the sensor a nice uniformly surface to shine at.
It's dirt cheap, works out of the box and interfaces nicely by usb or serial.