I don't believe that such a film could exist
and remain unshown thus far.
Well for what it's worth, the William Shatner 'Rocket Man' video was known by a small minority of folks for a couple of decades. Many had seen it, many had only heard about it, even more thought it was a myth, and most didn't even know it existed.
I have had a copy of it on an old VHS tape for over a decade (it really has to be seen to be believed). I think it's on the net these days though, as a video file.
In any event, it's possible for it to exist and remain relatively unknown for a while, but by now you have to figure it would have come forward.