Hydraulics are generally not used for a parking brake in the automotive industry because hydraulic pressure will, over enough time, diminish and potentially create a dangerous situation. Depending on the state of the seals and the oil this could be one day to several months. In either case, a cable brake is commonly accepted as the safest, and simplest, solution.
I have seen inboard rotors for a cable parking brake before - can't recall if I've seen one on a 930 transaxle or not. Best setup would be a simple machined/lathed rotor which bolts to the back side of the output flange (towards the transaxle) so that the suspension geometry (pivot points) doesn't change. A simple bracket can be fabricated which bolts onto the bolts for the output shaft cover to which a basic Wilwood cable parking brake caliper can be mounted. Shouldn't bee too difficult.