Using a android tablet embeded in my car
How about using widgets? Probably cheaper and would let you control all the functionality on the device itself, without any external hacks.

For volume control, you might be able to do with a widget similar to Volume Control Widget and for the power menu try an app like Button Savior or Shutdown Menu 2.1 (Both power menu apps require root access I believe)

EDIT:
Just realized that you might have issues waking up the device once the screen turns off. Found Wave to Unlock after a few minutes of googling. It uses the proximity sensor to unlock the device. This might do the trick.