I managed to reset my Origin password, logged in, and have only one game there, which was a freebie when I signed up (and probably the reason I signed up).
Still no sign of NWN in any of my game lists.
Still can't find it, so I just bought it (again) from GOG.
If anyone can think of any other online game repositories, please keep posting.
I've probably got a stash of games somewhere I've forgotten about!