It wasn't with National or at MCO, but after 9/11 we arrived late for a rental car reservation and were able to keep the rate we had been quoted.
In our situation, we booked flights, then our rental car. Then after 9/11, the airline dropped our flight. We decided to go on a flight the day before and I rebooked the rental car. Then the airline added back our original flight, so we wanted to go the next day again. No probelm with the airline, but with Hertz it in raised the rate by more than $100.
So, we just arrived late for our reservation. I called Hertz before our scheduled pick-up time to let them know we wouldn't be on the flight that was reflected in our reservation and they allowed me to pick it up about 18 hours later without changing the rate applied.
Because people do miss flights or have them delayed from time to time, arriving later for a rental car pick-up happens all the time. With Hertz, it ended up being no problem for us, but of course, you mileage may vary with National.
Best of luck -- Suzanne