replayguy wrote:

4) Some other user accesses the original mp4 on-line through a standard browser (e.g. FireFox) and the on-line application indicates there is "Kinovea inside".  If the user clicks that checkbox then the Kinovea overlays play.  (So I think what this means is that there needs to be a special browser player to make this all work.)

Yes, it would mean programming a specific web player.
Technically it may be an interesting challenge, but I'm not sure about the added value: if you want to share a video with the drawings overlaid, you can save it in this way and upload it as is. If you want to share a bare video with the drawings as a separate file so the viewers can recombine them later, you can also easily do so (although the combination will have to happen in Kinovea).

Still, it would be interesting in regard to the key images positions and comments, things that are lost when simply saving with drawings painted on. But the web player would have to be capable of handling them.
I am going to focus on the desktop application, but if anyone wants to leverage the XML data in any way for web oriented usage, you're welcome. The format is hopefully relatively straightforward.