My only experience of PRM was the initial trials at Minny. During those trials aircraft were allowed to take-off while a PRM aircraft was on approach. Since the localiser signal could be compromised by the departing aircraft, perhaps enough to cause an undetectable track deviation, (localiser bar would stay centered due to the bent beam) this did not seem like a good idea.
Have not looked at this procedure much but it would seem that, to be fully safe, it may not provide much increase in airport capacity. I would think take-offs should be restricted while an aircraft is using the ILS/PRM and that aircraft on the same ILS would need wide longitudinal separation to prevent loc/gs distortion caused by the first aircraft.