My neighbor had gastric bypass this past Nov, and has a card she got from her surgeon. We have just recently started taking her to TS restaurants again. Using her card she was able to get a childrens plate with selections she could eat at Whispering Canyon, and this past week she got a half portion meal (she brought half of that home for dinner the next day) of Shish Kebebs in Morocco. She has shown her card to the waiter and the Chef has come out and worked with her to put together a meal she can eat at a reduced price in both places.
We are going to Crystal Palace in a couple of weeks. I'm interested to see how she will fare there! I truly hope she isn't too tempted to overeat, but she's managed really well so far. I don't want to be a bad influence!
I really don't know how you will fare with the dining plan. I'm sure the Chefs will work with you, but I am not sure if you will end up paying any less than your regular dining plan credit for the meals. As a local, I don't have much experience with the dining plan but I do know that between helpful Chefs and sharing CS meals you should be fine at WDW. If you can, my neighbor also travels with allowable snacks like nuts, individually wrapped cheese, youurts, and her ever present protein drinks because we usually only eat one main meal when we go down for the day & she needs to eat more often than that. Hope that helps.