I've seen passengers jump on a 20 at Leazes Bowl and get off at the Ford Garage, heading to the P&R.
Might not be the done thing, but it happens. Waiting in the queue of traffic, you see them waiting to cross the road.
Pass holders of different types do it quite often.

Whilst I see where you are coming from, whether its designed as a P&R or not, its still a service designed to make money.
Scarlet Band put their tender in, the Council accept it. Scarlet Band then make the money to ensure the service is viable.