Spirit,
Adding Torque to a body will not result in the same affect as applying it at the joint. I'll give the example of a simulated elbow. The elbow has a limited range of motion of about 0-140 deg. If you are applying an external flexion torque to the forearm the elbow will flex, and the upper arm will remain motionless. When the elbow reaches 140 deg, the forearm torque will now cause rotation around the shoulder causing the upperarm to go up.
In actuality, the upper arm should, (if no resistive torque is applied at the shoulder), swing back slightly as the forearm goes up (as the center of gravity of the arm changes). When the elbow reaches 140 deg, the torque will not get transferred to the shoulder because it is internally driven not referenced to an externally applied torque.
The NVIDIA PhysX Manual, the revolute (hinge) joint has a motor parameter but I don't know how to access it from A8. I'm hoping that JCL is planning on adding lots more of the PhysX functionality to A8.