Err... not quite. B-RNAV and P-RNAV departures differ in that B-RNAV would require a turn below MSA to be based on a navaid. Not so for P-RNAV, where turns to waypoints can be initiated below MSA, once above 400'. There might be a navaid that is part of a P-RNAV departure, but that's most probably because it's an overlay of an existing B-RNAV SID. It doesn't matter if it's U/S: the only thing you need is the navigation accuracy (1.0 or less) required for P-RNAV, the aircraft has no defect make you P-RNAV unable and the crew has received P-RNAV training. As far as the flight management system is concerned, the U/S navaid is just another waypoint.
P