Disney has been driving positive guests to a location of their choosing in Orlando for Port Canaveral departures. Assuming they would do the same in other ports. You can have them take you to a car rental and the positive person waits outside while you pick up a rental car.
I you want to be sure of a quick/easy exit, you could rent a car, drive it to the port, and plan to pay for parking at the port for the cruise duration and return the rental after. More costly but then you’d have the car ready to go if denied boarding.