The purpose of a trial is to identify whether there are any shortcomings in the procedure. This would seem to me to include (but not necessarily be limited to) ATC procedures, pilot interaction with ATC, the actual procedure itself, and any shortcomings that may come to light with the equipment.
For any trial to be meaningful you would hope to engage as large a population as possible - the drug industry learned that lesson many years ago. The fact of the matter is that most "serious" IFR aircraft and users that would comply with the operational requirements of the trail are on the N reg. The fact of the matter is that neither the pilot nor the aircraft know (or probably care) what letter is on the side of the aircraft. Ideally, I would therefore want to engage as many people in the trial as possible. DFC has pointed out that the CAA could not do this without FAA permission. He may well be correct. That is however not what I said. Rustle asks whether I complained about other trials elsewhere. With respect for the reasons I have said I would simply prefer in ideal circumstances that the trail be as useful as possible hence my original comment - "excluding N reg aircraft is of course complete nonesense".
All that aside strangely you dont find too many G reg aircraft in the US or Australia or any where else for that matter, so I guess the FAA didnt think engaging the G reg would have made their trails any more effective.
PS - already done the playing, so it doesnt really matter to me any way, but I would have liked IO540to have played as well.