As far as the ones I have used, yes, all entrances are open.
ACO Grandstand is open, and quite a few of the others, including Dunlop(Goodyear).
Not sure on Parking, a reasonable amount will be open I'm sure
Arnage & Mulsanne are open, they are usually my first port of call on test day morning.
RLM hasn't broadcasted on FM in the past, but they will be available over the interweb.
In past years test day has been a nice relaxed way of getting a first view of the cars, probably better than the race week practice sessions IMHO.