Archive.org seems to do something like that with its downloads via torrent: it appears as a web-seed for the torrents, and I usually snatch the files from those seeds faster than DHT seeds usually show up (which kinda undermines the idea, though).

Frankly, I never saw any other seeds on Archive's torrents—but one of those I snatched has a ratio of 3 now, so presumably it does work as intended.