When I questioned Disney Dining about this, I was told that the official policy is that its your responsibility to get in and that if you aren't, your card will be charged. I called a couple times and was given similar answers though one CM said they are sometimes lenient if you call that morning from another park and explain the situation, though I wouldn't count on that. I'd call Disney Dining and ask (and make sure you get the name of whomever you speak with). And then I'd plan to make sure you are there before park opening so you can get in.