Silk might be what's called for here (Score 1)

Silk sounds close. www.silk-project.org. It's an OSS collaborate content management system. It has the WebDAV, and Active Directory/LDAP integration. J2EE based (comes with JBoss), runs on any OS that's on Sun's JVM compatibility list. I don't know if the WebDAV will work when run on a Mac... I don't think that's been tested. Silk still has a few rough edges, but it sounds pretty close to what's being asked for here.