I was very lucky a few times, before 9-11, to be granted jumpseat in the cockpit on various US legacy carriers, on a full flight. I had my FAA license, even though I flew for a major Canadian carrier.
Once , going into ORD in the jumpseat, of a MD-80 the rather flustered F/O got on the wrong frequency. A few seconds of radio silence ensued (!!) in some of the busiest airspace in America. I did YUL to ORD often, so finally I piped up, "try 119.2" . We had an immediate response from Chicago TRACON.
Anyway, now as a retired Captain, with 30 Years of service, I can't even enter the cockpit of our own aircraft, once the flight has begun..