First, I would head over the the DISabilities Forum. Those folks know a lot more than I do about Disney with allergies.
I always notated my DGD allergy when I booked my ADR's and then advised the CM upon checking in to the restaurant. The Chef always stopped by the table to make sure we odered what was safe for her. HDDR is a AYCTE meal, and I am not sure if there are items available that are not on the menu, so I do think it is best to be prepared to enjoy a limited selection unless the chef can manage to pull a meal together from what I on hand.