We have a lot of food issues to deal with. DS has ASD, and food texture issues. He is getting better as he get's older and more confident with things.
I have to say, most places will leave off a sauce, or swap a side for something that is listed as a side with another meal. (because, you know then that they have it in the kitchen)
Disney, I found, are among the best at making such swaps and making the food work for the customer.
Not sure I would ask for something entirely not on the menu, (that would suggest to me that I am in the wrong restaurant?)