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This feature is often called "whiteboard" and is mostly used in conferencing softwares. I could not find any collaborative editing software both having a web interface and not doing extra heavyweight features like VoIP (and often using Flash/Java stuff).

GuiHome does not need it anymore.

As for the software found, which could be useful to others, its websites are down and WP tells us there is no release since 2010.

So, until such a software is really needed I'm gonna cease my research.