+1
Or simply https://gersync.com/secret?<folder secret>
That's something I miss comparing btsync to centralized providers.
Sounds like pretty much work though, how would up/downloads be done? A javascript-based bittorrent client running at clientside sounds unrealistic, and a browser plugin to manage the transfer is sort of love downloading the whole client.
Since the main thing of btsync is it being p2p it also seems plain wrong to have additional servers between Web client and peers, converting torrent to simple GET.
Good idea anyway, I'll be following this one..