My take is that, to a large extent, the basic principles of counterpoint remain the same -- composing independent melodic lines which interact with each other harmonically. To that extent, the difference in contrapuntal music of various styles will largely mirror the difference in those style's non contrapuntal music. This means that you can combine a study of "regular" counterpoint, as well as a study of a desired musical style (including it's melodic and harmonic language), and adapt the one to the other. For example, in film scores, various genres are used, but often it is based on a large orchestral Romantic Era aesthetic, so you might look at Romantic conventions (a larger harmonic inventory, freer use of chromaticism in melodies), and incorporate those principles into counterpoint.
This might seem like a cop-out answer, but consider for example, that Fux's classic text Gradus Ad Parnassum, which describes old Palestrina-style counterpoint, was still used and highly respected, by Bach, Haydn, Mozart, and probably Beethoven. They merely incorporated its contrapuntal principles into their respective styles (whether Baroque or Classical).
I am not aware of a good source that already does this stylistic synthesis, especially a free one, however I did find the following (rather costly) textbook on amazon that might be of interest, as it seems to take a historic approach: Modal and Tonal Counterpoint: From Josquin to Stravinsky. That said, according to the table of contents, it is the final four chapters (25, 26, 27, and 28) that deal with Classical, Romantic, Serial/Atonal, and "Hindemith, Bartok, and Stravinsky" respectively. That's an awful lot of music history crammed into a relatively small space (each chapter looks to be about 15-20 pages).
Of course, you'll notice this book doesn't cover what you might call "popular" music styles - Rock, Blues, Jazz, Broadway, Film Scores, etc... Which often seems to be a problem with music theory textbooks in general.
Another source you might check out is Walter Piston's Counterpoint. One of the reviews notes that it briefly covers 20th century counterpoint, and another recommends it for Jazz musicians.
One place in popular music where I know counterpoint has been used occasionally is in musicals. I'm thinking, for example, of the end of "A Man's Gotta Do" from Dr. Horrible's Sing Along Blog, where you have a trio (Dr. Horrible, Captain Hammer, and Penny) singing in 3-part counterpoint, with the individual lines clearly differentiated. This style is common enough in musicals that it has its own page on tvtropes: Counterpoint Duet. Granted, it's not a theory textbook, but comparing some of the examples on there may inspire some ideas.
