Now I am thinking that it might be a good idea to come up with a new URL scheme that is backwards-compatible.

Instead of magnet:?ws= we could have just ?torrenthash=. Then all clients keep working as they are, but clients that want to join the p2p frenzy can implement a simple scheme to transform that into an actual magnet and then use that with webtorrent (or nonweb-torrent) to download the file from either peers or webseeds and verify the hash immediately.