There's nothing to stop you if you get there early enough. You are not allowed on the actual track, as is the norm. People with eventing dressage tickets will also be allowed to walk the course but do allow extra time to go through security again.
The gates open at 8.30 or 9.30 I think don't they? Not sure at all if they will allow you on the actual course (as in being able to stand in front of the fence and nearly pee your pants!)
The leaflet does say "once inside the venue why not explore the cross country course and see all its features"