I would favour the requirement that the service must be accessible, i.e. it should be possible for a wheelchair user to travel at any time where a service is advertised. I'd extend that to the likes of NatEx and Megabus, who must surely now be turning more people away due to the reduced possibility of using duplicates - if the service car is accessible then duplicates should be allowed to be non-accessible.
So for RRBs, a regular coach plus an accessible taxi departing at 1000 (or whatever) should be as acceptable as an accessible coach.
I know some don't like accessible taxis because their height makes them uncomfortable, but I don't like 3+2 coaches because my height and, er, girth, makes them uncomfortable. Both are legal and so both can be used.