Cockpit visit

Greetings to all.

I have run into a discussion whose answer is not clear. The question,
especially since 9/11, and its variant is:

Can passengers on commercial flights, during the flight, visit the
flightdeck (with captain's
permission, of course!)?

Essentially, is it the FAA or IATA that makes that determination or is it
air carrier policy?

Any links to the relevant documents would be greatly appreciated.

Thanks,
Charlie
PS. I posted this message twice under other threads. Sorry.