As announced by Rob, I recently started to hack an Abicollab backend using Telepathy tubes. It's based on the existing Write backend to ensure interoperability with Sugar.
See this screencast where an Abiword document is shared to participants who are using Abiword and Write.
My code is quiet rude and still need a lot of cleaning and polishing but that's a big first step. Hopefully, the futur improvements in Telepathy and Abicollab would make possible a rocking desktop integration of the collaboration features in the near futur!