According to the press release, this footage-less trailer is one minute long. I assume it'll be a retrospective. It's short enough to serve as the "prologue" to a normal 2-minute trailer. Just cut them together so we get sort of a past/future thing, with Matt Smith's voice or John Hurt's voice providing the bridge transitioning to the actual 50th footage.
That would be awesome. And I'm sure someone will do it on Youtube after the fact, showing the BBC what they should've been doing all along.