Is it possible to, using javascript, either create video data on the fly, or more likely download pieces of video data from various sources and feed them to a flash player instance.
Even better, is it possible to feed a html5 video stream from locally executed javascript?
Short answer: no.
Long answer:
To pass the data directly from JavaScript into Flash, such as by ExternalInterface, you would need to serialize the video data as a byte stream. This means JS must be able to access video frame data, convert it to a String, and send it to Flash.
Serializing video data in the browser with JS would take some pretty fancy footwork; given the way HTML5 video works, I don’t think any mainstream web browser supports this.
As a suggestion – you can use JS to feed URLs of video files to Flash where Flash can open the connection and access the data itself. In general Flash is vastly better at manipulating media data like videos and images. If you’re already planning to using Flash you’re better off cutting JS out of that process anyway.