Its been stopped now, unfortunately. But my Father did do it, many years ago.
You used to be able to ask a Policeman at the side gate to sign the visitors book. He would signal (later radio, but it was a long time ago when my Dad did it) to someone inside, and another Police Officer would come over, let you in and escort you across the parade area to a side door. The door led to a hallway, and a small room off the hallway had the book. You signed the book and were then escorted back to the gate.
OK, not quite positive proof, but I have no reason not to believe my Dad, and he did not just tell me it was possible, or that someone he knew had done it, he told me that he had done it himself.