Hollywood studios has a lot of bars that are part of restaurants that do not require an ADR. I've walked in, sat at the bar without getting any drinks and ordered from the full menu with adjustments for allergy needs. I don't know if sitting at the bar fits the people your traveling with but if it's an all adult trip, it might not be out of the question..
Also my best spontaneous trip advice for an adr, is start checking the restaurants you want at 11pm the night before. People who are hording reservations have to cancel them by midnight or they risk getting a no-show $10 per person fee when they don't use them. You may not get the exact restaurant or time you want, but you'll find something for at least one of your meals that will help with your dietary choices. Also along the same line, don't assume you can't walk up and get right in, early meals (11am for lunch, 4pm for dinner or really late (right at the time they close) I found walking up to be most successful.
I never tried it, but let the quick service place know you need a low sodium choice and let them help you. even quick service have a "chef" if you request to talk to them that will come out and speak to you about their food options.