In reply to richprideaux:
I don't know the legal position, but I am pretty sure that rights of way can be, and are, suspended for the duration of the rally. However, like Caterham, I go to the rally and have never been challenged when I've used a bit of initiative walking into forests away from the official, ticket-only entrances.
So I don't see that there need be any particular problem for a discrete runner who keeps off the tracks used by the rally and is polite and tactful with any marshals they should bump into. The route of the special stage is easily enough worked out when in the forest. Marshals will only be there for the day of the rally, so they will naturally be concerned about your own safety and, sadly, owing to some recent incidents, competitors' safety.
Local access roads will usually be closed from mid- late-afternoon the day before the rally, but I don't know when in the case of night stages. If you are a local resident, you should be able to obtain a resident's permit.