They do no substitutions. It's on the menu and they will tell you that when you call.
It makes sense place like that wouldn't allow any.
Forget substitutions, they wouldn't even leave something off the plate because it "ruins" the presentation.
As a chef who has worked in my fair share of high end restaurants, I can appreciate not willing to change menu items.
However,, when a customer has an allergy, it is best to accommodate - there are always exceptions to rules.