Most, if not all, European countries now legislate against flight deck access during the flight except by specific people.
The USA also dictates the same to foreign airlines flying to/from the USA as it does to its own airlines.
Other parts of the world of course may or may not have such strict rules.
And there will be occasional Captains who buck the rules (at considerable risk to their careers and possibly incurring the wrath of their CAA and Government on their airline).
At best I would put the chances of any passenger being allowed on the flight deck during flight as 'unlikely'.
Of course .. in my Piper Archer it's different. Not much choice with the cabin layout
