My flight was cancelled once and Delta had to route me through a completely different city. I did not arrive on time or with the correct arrive flight number I still got to ride. If you want to take it this far… you could look up flight numbers for that day for a specific airlines. You do want to add the correct flight info for when you return so you get picked up in plenty of time.
If you have a Disney hotel reservation, then you should be able to ride Magical Express from the airport on the first day of your stay. The one time I arrived without Magical Express tags I stopped at the check in desk at the airport and the desk clerk was able to find my reservation and send me to a bus.