It depends on how far off menu you need to order. If it is making a small change to a dish, no problem. If you are expecting the restaurant to make you your own special meal than that may be a bit more difficult. Disney goes out of their way to please diners but they can only do so much. It also depends on how busy it is, which with Disney is almost always very busy.
Like the PP said tell us more and we can help more.
B.