For its very first music Samplethon, the copyright clearinghouse WhoSampled got together 20 music producers at the Great Escape festival and set them loose on an ultra-rare set of vinyl records culled from the private collection of Imagem Production Music, the modern day iteration/owner of the Boosey and Hawkes Recorded Music Library and its Cavendish sublibrary. The cullers themselves were five "known" producers, ranging from Jonny Cuba from SoundSci to My Panda Shall Fly's Suren Seneviratne (OK, not too known), but it's the private press nature of the collection that really interests.
Imagem and Boosey and Hawkes before it guard a vast archive of a very particular variety of record. Private press releases were typically never meant to be sold commerically, but are rather the product of collectors and others that just want to have a physical version of some piece of music for their very own purposes, whether its archival enthusiasm, pure vanity, or to supply proto-Musak. If you were a music maker before Bandcamp (or MySpace or whatever) and wanted to DIY a release, a private pressing was pretty much your only option.
In the case of Boosey and Hawkes' Cavendish library (which is still being marketed by Imagem and, yes. it's complicated), the records were made for the purposes of radio broadcast backgrounds and theme songs. They're labeled by number not creator, and come with a brief description on the back of the record's sleeve. You can see why this stuff might be especially tantelizing for a crate-digger producer/DJ, just given that most of it's never been heard outside of some 1950s radio show and certainly never sampled. That doesn't mean it's good or desireable for sampling in music, but that's why the contest used pre-dug material. The Samplethon intro video above might make it more clear.
So, the actual participants are just given the good stuff, or at least the stuff determined to be worthy of the competition:
So, yeah, it's more competitive than collaborative (which I think is part of the "hackathon" definition) and coding tends to involve using existing materials in new ways, but who am I, the metaphor police? Still: an interesting idea.