In the air no chance, but on the ground I think is quite possible. Last year myself and a large number of others on the top deck of a UA747 got a nice tour of the cockpit while still on the ground in Australia. Pilot told us about the instrument capabilities, showed us the crew rest area, etc. It was real interesting and everyone really appreciated it. Bolted up like a vault when it was time for take off, but the doors are open prior to flight anyway (and after) so I imagine it wouldnt be too hard in many cases to get a view while on the ground.