I think an idea of mine is related: index clips from various sources to put together an entire movie. Or index quotes posted to various forums or quote websites

I think an idea of mine is related: index clips from various sources to put together an entire movie. Or index quotes posted to various forums or quote websites to reassemble the original book. No one hosts the full content, all is fair use, and the index is also meaningless by itself. Though I suppose you'd get a similar conviction as The Pirate bay which also hosts no copyright infringing content. Your idea would be a bit more 'direct' in that the parts are hosted by people actively contributing to have a copyrighted work in full rather than linking to random third parties. A lot more reliable, at least until it catches on and contributors start receiving desist notices.