I've mentioned this informally to a few people, but it came up again in discussion and I thought maybe I'd bring the idea to a wider audience.
TowTruck (https://towtruck.mozillalabs.com/) is a realtime collaboration framework developed by Mozilla (but not firefox-specific). It provides the real-time communication infrastructure for collaboration on websites. It could be an interesting foundation for something like Flow.
TowTruck has a plug-in architecture for custom editors. Visual Editor would use this to handle the synchronization within a visual editor widget. Tow Truck would provide the "find a friend", "real time chat" and other collaboration plumbing. Flow would provide the overall UX design for the collaborative process, especially the non-real-time archival parts.
Anyway, I think Tow Truck is an interesting project, and it would be interesting to try to avoid reinventing parts of the wheel. I'm interested in others' thoughts. --scott