Is the flight deck closed to visitor's in flight, a US reg? The reason I ask is when I had the good fortune to travel aboard BA's Concorde, from LHR to JFK back in 1998, the captain invited everyone on board to take a turn 'visiting' while high over the Atlantic. I would say about a third of the 100 passenger's accepted that invitation, the rest must have been privileged "regular's", who were too blase to take advantage of that incredible experience. The highlight was seeing the amazing "panel gap" between the flight engineer's panel, and the wall, which did not exist when the aircraft was on the ground, all due to the high heat and expansion from the supersonic slipstream.
Another flight deck experience, was on UAL DC-8-61 from EWR to DEN, back in the late seventies...we were also delayed at the gate by weather and ATC, and the captain invited us up for a peek, and graciously answered 'tons' of question's, until it was time to pushback.
And my last flight deck experience was aboard an AE Shorts 360, where I actually got to sit in the jump seat from JFK to PVD. The non-pressurized boxy fuselage featured a roomy lav that the passenger's on a 737MAX could only dream about using...