HOAX : Has the ending of Lost already been filmed?

I've heard from 3 different sources a version of the following, now I'm pretty sure it's just a case of "Chinese Whispers" and a some Urban myth, but you never know, and it would certainly explain why Matthew Fox has said many times in the past that he knows the end of Lost.

Well, the sources are reporting that at the end of Season 3, Matthew Fox and Malcolm David Kelley filmed a secret scene that would be used in the Series Finale. This was to overcome any issues of the actor playing Walt (Malcolm David Kelley) from aging too much. Now the various reports don't make it clear if this is the end scene of Lost or just a scene from the Series Finale. They also say that this was done to throw off the spoiler hunters so that they may have thought it was just some random or deleted scene. If this is true then not only do the actors and writers know the ending but also the crew who filmed the scene. If we get any more info on this I'll pass it on.

Do you think it's possible that the ending has already been filmed, regardless of whether the above is actually true?. I guess well find out in just over a year.