Only the fact that a campaign has never been hosted on Bnet should tell you that it's not possible.
Project Warsoc (one of the hosted projects here) however tries to adapt the online mode so people can access more features (such as 13+ players in 1 map or campaigns in multiplayer) without using bnet... For now it's still a wip
It might be possible but to do that the people must all have the maps and all in same location before they start playing map 1. Let me test my theory..... soon but not now.