Hi,
For what it's worth, number of years ago I saw at SIGGraph (possibly 2006, in Boston) a development from Caligary's trueSpace, a 3D modeler (http://www.caligari.com/), that allowed people to work synchronously and simultaneously yet remotely on models that lived in a shared 3D space, without recurring to any 'baton-passing'. This shared space was called truePlay and though it didn't have A/V/chat for communication, coordination and collaboration, I saw them using it with NetMeeting - http://www.letsgodigital.org/en/news/articles/story_7710.html.
Caligari was later bought by Microsoft and then... who knows but at the very least this proves a point, namely that co-working on a same file was and moreover should be today perfectly possible at least so, with the right tools and in an appropriate environment 😉
Cheers,
. Diego .