Probably not. That's a slow time of year and lunch is usually not as busy as dinner.
I went in August this year for a late lunch at Chefs (had a reservation) and were one of 3 tables in the place.
I doubt you'll walk into LeCellier or CRT but you will probably be able to walk into several other places. Try stopping at guest services when you come to the park in the morning and ask about dining for lunch - if they still do that, it'll save you time from wandering from restaurant to restaurant.