Actually I have no problems with running a trial; I can see that ATC staff need some practice too. This stuff is going to be totally alien to just about all of them.
It's a good idea for pilots too (those few that are flying TSO129 equipped planes).
What I don't see is the G-reg restriction. I don't see any practical or legal reason for that whatsoever, and have never heard anybody come up with anything remotely resembling a reference. It's particularly daft given that once the IAP goes official they will get plenty of N-reg pilots flying it; most of them do have a BRNAV GPS and a current database (if they don't then why the hell did they do an FAA IR and go N-reg?)
One of the justifications given by the CAA for the trial is to find out if the style of a GPS approach, specifically the way the distance shown on the GPS is to the next fix and not to the MAP, is going to be confusing to pilots. They have made much of this. BUT there are plenty of non-precision VOR/DME approaches around the world where the DME doesn't read zero at the MAP; often nothing like zero. Have a look at LIPU for one of countless examples; I recall (not having JV3 handy) it reads something like 21D at the MAP. The pilot does have to read the approach plate! The CAA is assuming he is an idiot who can't read.