I don't know of a torrent client that will allow me say "download at least these files of the torrent, and the 100mb of anything else that's the most likely to

I don't know of a torrent client that will allow me say "download at least these files of the torrent, and the 100mb of anything else that's the most likely to help the swarm". Downloading the chunks that are the rarest first is different than downloading only rare chunks - although it's good that some of the infrastructure is already in place.