Ive seen numerous instagram posts of people doing this on foreign carriers.10 yrs ago I had one 9E pilot claim he got to physically sit in the jumpseat of an Air France from CDG to ATL, as in the flight deck. I don't know why he would lie about something like this, so maybe it's true or maybe not?
One of them was an SAS FA who was trying to non-rev with her friend to Miami. The only seats available were in the flight deck. They had some pretty good footage of herself, an SAS FA, and her friend, a non crew member, sitting up front while on approach into Miami. It appeared that mid-flight they got to sleep in the crew rest area.
Another post was a buddy who works for one of the big 3 in the flight deck of a European carrier as it made its way across the Atlantic.