Notwithstanding notsosmartashethinksheis' doubts, this is not the urban myth... personally I have never heard an urban myth of a captain locking himself in the lavatory. I am familiar with the story of an entire crew locking themselves out of the flight deck.... this is not that story.
It is possible that I was told a tall tale- however, as (so far) I have heard the same story from
(1) The entire cabin crew (separately).
(2) The engineer who repaired the door.
(3) The base captain.
(4) The captain who was supposed to be taking the aircraft over at the next turnaround.
I could name the airline, the flight number and the captain, and specify the date- as I have no wish to embarrass those involved (except face to face), I will say only that this happened in the first week of November 2004.