Earlier quoted context omitted.
So, you're suggesting that SoundCloud seeds all the files instead of offering the download option. Perhaps this could work if "play-in-browser" vs "download" ratio is not too big.
And play in browser could be handled by something like torrent time, no?
I haven't actually kept up to date recently on the project, but the ML is super active. I believe they now have a working localforage backend to store torrents - the implementation already works flawlessly, and there is a hybrid client in the same node package family that can seed the same files to both webtorrent cleints and traditional UDP clients.
But it is extensionless and seamless and just uses webrtc data channels and websockets.