Having flown over 500 of these fully coupled DGPS approaches, to a hover, in conditions as difficult as 25 knots direct downwind, at angles as steep as 9 degrees, I can tll you that yesterday's autopilots are capable of keepng the aircraft within about 3 feet of the center of the beam, so nothing technically stops the effort. The current nav systems are capable of accuracies within 1 meter, without a ground station, and one inch with one.
Shawn tells it right, the failure of regulators to create the criteria, and the failure of operators to ask for it is where we have ground to a halt.