If you are staying at the Fort I would say try to walk up and see if you can get in. If they can't take you you can always buy some cereal and yogurt at the trading post or hop a boat to another resort or park to eat. If you are staying elsewhere and would have to travel to Ft Wilderness then I wouldn't recommend risking the amount of travel time against the prospect of hungry bellies.
Keep trying for the ADR, people change their plans all the time.