Gatewaying separate vBulletin sites

[macbbs]

We are interested in finding a way that several independent organizations can each operate its own vBulletin site for their own users - but share certain sub-forums so that if a user on one one site posts to a shared sub-forum, that post will be seen by the users on the other sites and can be responded to from any of the sites.

As an example, think of independent organizations in New York, Chicago and San Francisco, each of which operates a vBulletin system with many sub-forums for local issues but- in addition-each system has a number of shared sub-forums for national issues.

Is there an existing hack or plug-in that would make that possible. If not, do you think it could be done easily?

Initially there would be three sites but more might be added later.