It's a shame that cockpit visits are not allowed. I cannot recall any hijacking beginning with a polite request to CC to go up front.
Although perhaps the following wasn't a great idea. My oldest brother - maybe 20 at the time - during a VC-10 cockpit visit was invited to sit in one of the front seats (presumably on the right), put his hands on the yoke, and then they disengaged the auto-pilot. I remember him saying that it went into a *very* gentle bank at which the captain (or FO) swiftly resumed control.
Madness perhaps.
Before inevitable comparisons with Aeroflot 593: the crew *knew* that the autopilot was disengaged. And were monitoring very closely. And (somehow?) knew that my brother was doing his very best to keep it level. He'd already set his sights on an aviation career by then.
Years later, brother ended up sitting LHS on Big Airways 777s.