This may have come up before but I was looking through the transcription notes in "Unaccompanied Django" and reading the section on "Nuages"; the extended solo version. You say, Michael, that the film was never made but it was and it is called "Rivages de Paris". However, other than having Django play on the soundtrack, it is a pretty boring film of Paris river trips.
Also, I doubt it was recorded in the dark as you say because when you watch the film, you could be convinced the music often matches what is happening on screen.
Sadly, Django is not seen at any point. ~X( Here are a couple of stills from the film.