I'm sure it's never been allowed- and it would be unthinkable these days but I certainly remember visiting the flight deck on a UK charter airline 737 in the early 80's and sat in the captain's seat for at least 15 minutes whilst Sir was in the galley chatting up the hosties. I even turned the aircraft using the heading knob!
Now I am in the LHS I wouldn't dream of allowing such a stunt- but it happens from time to time- people get carried away with the occasion and it only takes a moment of madness and that's an ended career. There have been numerous such stunts over the years. The problem nowadays is that you can expect any photos to be on Twitbook within minutes of landing.
As a trainer, I always remind my new captains to seriously think what they say on the PA or RT- in the event of an incident or whatever- it only takes a flip comment and because some idiot will always be recording everything, that comment will possibly make itself into the media and open up a world of trouble!
We live in a different world to that of 30 years ago. No more welcoming people into our rather secret world any more.