The distinction of using a dash-cams that is not fitted by the dealer is the same for anything generally speaking fitted to the car in that it’s not approved by Motability unless its fitted by the dealer.
Granted roof boxes need to be insured with the car so ideally you should approach the car supplying dealer to fit one but I’m sure there are folk who have gone to Halford’s and had one fitted without a thought of notifying Motability or the insurance implications.
If fitting a roof box to a car is classed as a modification to the car then surely you would need to notify DVLA of such modification so from my view fitting a purpose made roof box to the purpose made roof rails is not a modification but you can only be sure 100% by going through the dealer or Mota.
My dash-cams too are connected to the cars 12V accessory sockets so no modification to the car have taken place however as you say if you hard wire them yourself or is done by any one else that’s not the dealer there is an insurance implication with regard to fire risk but besides that even using the accessory sockets there could be implication of getting feet trapped on the brake pedal as example with badly routed dash-cam wiring so you could say its a grey area like fitting a roof box.
I don’t know if Motability have change there rules on dash-cams and how and who fits them and Motability advisors themselves have discernment on the matter but end of the day having anything fitted to the outside of the car I would go through Motability or your dealer when ordering the car.
BTW with regards to dash-cams and being hard wired so there always on I know the VW’s I’ve had in the past you can change the way the accessory sockets are powered i.e., always on or only when the ignition is on by taking the accessory fuse out of its holder and reinserting in a different configuration but never done it myself but know you can and I have no ideal if that’s a modification.?