As requested: Basically, the gist is that you accept the upload via a local JS file that acts as a conduit. You then turn the dropped / selected file object into a blob object and transfer that blob JS file that lives on S3 (using postMessage and a hidden iframe). That JS file on S3 is what actually performs the upload and tracks the upload progress. On progress events, I send back postMessage payloads to the local JS file to show updates to the user.
I'd love it if you could write that blog post. The technique sounds like it could be used for a lot of things, including communication in the other direction, i.e. downloading images meant to be drawn on a canvas from servers that don't support CORS.