Prior to 9/11, civil controllers working for NATS were permitted to take free 'familiarisation flights' with some airlines for 'liaison' purposes. We were issued with letters of introduction to hand to cabin crew in the hope the captain would grant access to the flight deck; the Captain would be informed prior to the flight to expect us on board.
In some cases (usually IT flights eg Britannia, Air 2000 etc), the check - in agent would be told to expect you in which case you were often not even allocated a cabin seat (which they could sell of course) and travelled out and return in the jump seat. On all flights you were only permitted a single night stopover if there was no return flight the same day (eg Air Malta to Malta).
Of course, this was all done officially, whereas it would seem the letter above was done on an individual's own initiative.