There is not only the motability rules to consider but also road tax rules.
Firstly the motability rules are to allow a named driver to go to the shops for the disabled person without the disabled person in the car.
This ties in with disabled road tax rules which states all journeys made need to be for the benefit of the disabled person.
Another person using the car and it being available to you does not meet this requirement.
It sounds very much like you want someone not in your household to benefit from a motability car and say having it available to you meets the needs of the rules.
Well no that neither meets the motability rules nor road tax rules.
Be warned abusing road tax rules and getting caught can end up in DVLA seizing the car.