Man this is a nice post! I'm amazed that you were able to think it through in a short amount of time, but then again you have been working on it for a while.
Reminds me of when I used this to grab f4m manifest files from flash players. Used it to rip of some video for a project.
You did it in the modern way though and in way less code and complexity.
I wonder how further you could probably go with this? It has potential for streaming video by messing around with the chunks. Vatic has managed yo use video fragments to annotate certain areas of video, so using something like what you made in conjunction with YouTube might make a better annotation system than what they have now.
Of course I could just make some new code in YouTube's source for the same result :P. Sorry if I complicated my explanation. I just meant that what you made is a simpler way of trying to do video annotation than messing around with YouTube source.
I've not tried this, but you can try following the instructions in this youtube video.
The video you linked is being broken up into small .ts files. You (supposedly) can use Grab Any Media to get those files, and then merge them back into an actual video file with FFmpeg.