I remember an interview Michael Kamen did with either a (British?) movie or a music magazine circa 1989-1990. In it Kamen mentioned a piece of classical music he was fond of that he always tried sneaking into the films he scored. In particular he mentioned the 1989 James Bond film Licence to Kill.
I cannot for the life of me remember the name of the piece. It wasn't Fur Elise which I know appears in the Bond film. I seem to recall it was something like Orpheus and Eurydice (though just as likely it wasn't). The title had multiple words, and was not as easily recognizable a title as Fur Elise. I had to go back to the magazine to doublecheck the title.
In an interview with David Kraft for Soundtrack! magazine (vol 7 no 26), Kamen talks about the film The Next Man. He said it was temp scored with the music from Black Orpheus, which at one point the makers were considering buying and using as the score for the completed film.
Kamen says Black Orpheus was one of his favourite film scores - so he was faced with actually replacing it for that film. He adds that he tried to retain that feeling.
Orpheus was a jazz rather than classical score as far as I recall, so not sure if that's what you were thinking of, but hope that's of some use...
...In the same interview he says that Nascimbenes's score for The Vikings was an early influence on him but a quick scan of the article doesn't reveal him mentioning any pieces that he actively tried to slip into other scores, as far as I can see.
