I'm sure you won't have trouble, especially in the Grill. I am another Grill advocate, by the way, perfect for a quiet romantic dinner for two. Food not perfect, or up to the other restos, but I have always enjoyed it.
I for one hope these advanced reservations don't catch on. Except for groups perhaps, they don't make alot of sense, and I like to be a bit spontaneous. Oh, and each passenger is guaranteed one booking in each of the specialty venues, as I understand it.