Anyone returned National rental car at Dolphin when closed?

[mouseclick1]

We are planning a Universal/WDW trip for Christmas. We will be at HRH from 12/22-12/25, then at BWV from 12/25-12/31. I thought what we would do is rent a car at MCO and drive to HRH. Keep the car until we move tp BWV, then return it at the Dolphin on Christmas day. I checked this out at National's site and a message came up that the rental center is closed then, and to find out what special arrangements we need to make at the rental counter at MCO. I would rather know ahead of time how we would return the car if the center is closed, so...has anyone done this? What would we need to do? We have always used a towncar in the past, so this is new to us. I would rather be able to move from HRH to BWV when we feel like it, and not at some prearranged time. Thanks!