Agitate browser implementers to add support for magnet: URIs and you can do exactly that without needing to add new attributes to HTML. :)
There's a bug for firefox here: https://bugzilla.mozilla.org/show_bug.cgi?id=528148
There's a bug for firefox here: https://bugzilla.mozilla.org/show_bug.cgi?id=528148
But the browser supporting identifying files via hashes is vastly different from it fetching files via BitTorrent.
I think it makes more sense to pursue each separately. Even though magnet URI support will deprecate any existing implementation of hash support.