Hi Marco,
wie discussed it here in lengths, I may give a short summary:
popcorn.js is really cool and would be the perfect shortcut in the Schnittserver project. Implementing it seems to be relatively easy, as the Schnittserver serves all the WebMs as "previews" or proxy clips from the raw footage it hosts. As soon as a user is choosing to finalise / render a flat video, we would just need to transcode the JSON from popcorn.js into MLT XML and we can feed this right into the existing toolchain. So basically except the conversion JSON to XML all is already in place.
/Manuel