It's understandable that the avionics engineers are jittery. It flies directly in the face of everything they know, namely that aircraft avionics MUST be certified and uncertified kit is the work of the devil, sure to fail and bring the aircraft crashing to the ground. I've met plenty that think like that and it's not surprising because all their training reinforces that view. When I was first looking at moving from owning a C of A aircraft into the Permit/LAA world I was assured by my then engineer that it would all end in tears.
It's not surprising, either, that there were no C of A aircraft in the trial. Even with the minor mod fee waived, it would be an expensive job (simple task, time consuming paperwork), with no certainty that another expensive job would not be needed to undo the mod afterwards. Add that to the engineers' reluctance to recognise the validity of uncertified kit and it just became too hard and difficult.
The situation should be different now, with the trial completed and the LAA allowing all its aircraft to continue using uncertified GPS into ADS-B out. It probably needs some sort of formal statement from the CAA that the same will apply to all C of A aircraft.