Well your right J2F about “would have a better system” but I think they think as only one third of Motability cars are driven by disabled drivers with the other two thirds being driven by carers of the disabled they (Motability/RSA) think its not worth there while.
Last year I had the same, two weeks to get a stand-in car with adaption and hoist but they subcontracted the hire out but only because (I think) I refused to let them take my own car away before getting a hire car.
Enterprise are IMO useless and I had one guy ring me up about 12 PM to ask what I needed but first he went through the security checks with name, address, postcode, Motability number and on and on, spent more time on security than he did about a suitable car and when I mentioned the steering ball with remote controls he said “we don’t have anything like that” not that I think he understood what I was talking about.
He goes away saying he can’t help then low and behold at 3 PM he rings again and starts again on security questions and I pointed out that he rang me, not me rang him, so you know who I am because you asked the same questions just three hours earlier and I don’t live in a public phone box.
By this time I was livid and told him in no uncertain term to go away and within an hour a guy rang from an independent hirer who actual knew what they were talking about with regards to disabled driving and disabled drivers and I got the car the next day so about two weeks to get a car all told and 1 month before every thing was back to normal with my own car.
I do hope they come up with something for you soon.