Timbo, from a biomechanical perspective the most efficient placement of the hook is closest to the body's Center of Mass (COM), which is located in front of the sacral bone (or S2). This happens to be the approximate location of the hook for most harnesses. The further you place the hook away from the COM, the more muscle action is needed to stabilize the body while on the wire, or more elaborate support mechanisms will be needed. If you would move the hook placement higher on the trunk, you would need more muscle activity of your lumbar extensors and other back stabilizers to stabilize your spine, and over the course of a day this would probably lead to more low back fatigue/pain.