Although the answer is most likely to be 'no', there are still some crews on certain airlines who may (very small possibility) allow a flight deck visit in flight. I was lucky enough to be invited up post 9/11 to a 777 flightdeck on a hop across the pond as my friend (who was the captain's intended target as my mate's old man was a former colleague of the captain) was asleep. "Anybody else want to come and have a look?" "Me! Me! Me!" Great fun it was too until the chief purser came in and informed the boss that the garbage in first class were nervous about the visitors they had just seen.
More recently, that lanky lout Clarkson blagged a jumpseat on a scarebus for his recent programe about the jet engine, along with his camera crew.
Not that I would suggest joining the BBC is a good way of trying to beat the odds for a flightdeck visit, but it isn't beyond the realms of possibility.
No doubt I shall now be flamed vigorously as some kind of irresponsible idiot, but somehow I don't believe this post will lead to fligtdeck doors being flung open willy-nilly for visits. It's not my decision to do it or not.
