This one is a bit tricky.
Depends on the definitions of "rarely" and "heard". Does it relate to how obscure the score is in relation to the amount of renown afforded the film?
In that case, I would nominate Last Year at Marienbad. I'm a longtime fan a classical organ music. I was struck when I first saw the film around 2001-ish that it was perhaps the only film ever with a through-composed classical organ score. (I welcome additional items for that list) It adds a great deal to the film, even though according to the BFI book about LYAM, the choice to record an organ was initially a cost saving measure. It may not be the best mid-century modern organ music, but it isn't complete dreck, either. The composer, the brother of the main female lead, was supposedly a composition student of Messiaen. Thus began a decade long question to either locate the score, or find an out-of-the-box organist interested in doing an improvisatory re-recording or re-interpretation. Which would surely be a reasonable task for an academically trained and talented organist.
I attempted to contact, or contacted, at least 5 entities which were possible leads on the score, and several important (wouldn't call them "major") labels for recorded organ music. None of the labels found it interesting enough to even reply. The most promising score leads - the commercial entities which seemed to inherent whatever became of Mondiamusic, the publisher of record - didn't get back to me. The most promising was a professional french musicologist who claims to have contacted Seyrig's widow. (That of the composer, not the star. As so often seemed to be the case with french cultural figures of the mid 20th century, he died in a motor vehicle accident) He said the widow thought the score might be in the attic of her summer home, and she would check in a few months. Alas, that never went anywhere either. Some joker claimed to be willing to sell me something for some high amount of euroes, but he was really only talking about a 45rpm that had 4 short excerpts of the music. The Oratoire du Louvre, the Protestant church in Paris where the music was recorded, at least responded to say they did not believe the score was in their archives. I never tried to directly contact Alain Resnais, and I sincerely hope that perhaps he has a copy in his personal archives that will be available once his papers are donated to some academic institution after his passing.
So, that's where things are now, over a decade after I started this quest. I'm a fairly good enough classical organist now - I'm in the very rare category of people who dabble but have never been professional church musicians - that over the next 10 years I could probably adopt, by ear, some of the better vignettes and record my own mini-version of score.