There was a series of 'whale-watching' charter flights using Icelandic 747s a few years ago. From memory they always parked on the 'six-pack' Eastern apron. Others have been one-off football charters, sub-charters and diversions.
Hesitant though I am to contradict vintage ATCO, I seem to remember the first 747 visitor was in Caribbean Airways colours. Didn't they have some link with Freddie Laker?
Over the years, I would imagine there have been a couple of dozen 747 visitors.