This has been a problem for WAV customers, getting a hire vehicle. Motability have taken great steps (for WAV customers) and have now made the WAV supplier the point of contact for all issues, service, repair, MOT etc. This has put an extra burden on the WAV suppliers as they are responsible for supplying hire vehicles and acting as middle man between customer and various companies involved, but is an excellent development for WAV customers.
For customers who use adaptations, roofbox and hoists, the issue of requiring a rental WAV still exists, as you have posted. The problem being that the fleet of WAV hire cars available will have diminished as the WAV suppliers have their own fleet of vehicles to loan to customers. This is an area that Motability need to look into and resolve. I did want to put WAV customers mind at rest over this issue.
The only thing to do, frustratingly, is to keep phoning Motability.