In general most restaurants are pretty good about easy, basic modifications, like you said, sauce on the side. But substituting one main ingredient for something else is different, in my opinion. For one thing you are impacting an entree pricing by doing that.
We generally only book a restaurant where everyone can order something they will enjoy with, at most, a simple modification. I know Disney has some amazingly themed restaurants, but if the food doesn't appeal to everyone, it is a no go. I will never get to experience Skipper Canteen for that reason