I am talking a lot to myself here…
Anyways it seems a screen capture feature is already built into the next version of Chrome (v26 Canary). It does seem horribly slow on the local machine probably because it’s intended for streaming and the screen image you see has already been compressed as a video.
But it seems if Syphon was ever implemented it would be here in this API. It also is contrary to what I thought about screen capture being too much of a security risk.
(you need Google Chrome v26 and you need to set the flag –enable-usermedia-screen-capturing)