I think the Car Care Center will pick you up and take you back to your resort, as long as you take your car and leave your car during their hours. Most of the rental places operate from 8:00 to 4:00. You can leave a car there after hours, but would not have transportation home.
We have rented a car at the Dolphin from other resorts, and returned it after 4:00 with the valet people. We take the bus to the Studios and get on the boat (or walk) to the Dolphin. When you return the car, reverse the process. Also, there is bus service to the parks from the Dolphin. You could take a bus from there to the MK and monorail back to the Poly.