Do you know any fairly senior pilots? I know of a person that got a jumpseat ride all the way across the pond in an A330 with a well known, long established, European carrier. However it took a lot of paperwork and a lot of cooperation from that pilot and the airline. That may be a good place to start.
In terms of just rocking up and asking if you can sit in the cockpit when you board your next commercial flight then the chances will be next to nothing apart from, as FT says, trying a carrier outside of the US/Europe.