Disney's Magical Express (DME) provides transportation between MCO and Disney-operated WDW resorts, regardless of now you arrive at the airport. I've returned a rental car at the airport and then used DME (when I had a rate that made it much less expensive to return the car at the airport instead of at WDW).
Contact Disney's Magical Express Guest Services at 866-599-0951. The agents there know more about DME than typical WDW agents.
Another thing that people do is to provide flight details for a flight that gets into the airport about when they want use DME. It really shouldn't be necessary to do this, but it's not cheating. It's a way to get around WDW reservationists who don't know that you don't need to arrive at the airport by air. Disney only uses the flight information to plan DME capacity and staffing based on when passengers are arriving.
However, there's a more convenient option...
I would recommend looking into returning your car at a location on or near WDW property.
Drive to your WDW resort. Check in. Unload your car. Then only the driver has to return the car. The local rental car company location will drive the driver back to the resort.
It's much more convenient than having to go to MCO and wait for DME.
For more about car rental locations on or near WDW property, see the "sticky" thread on this forum,
Car rental locations for WDW...
Then use DME one-way only to get to the airport at the end of your WDW stay.