Basically I'm after three-way master replication i.e. any box can be the "master" at any arbitrary time, if that makes sense; develop on Mac, test on FreeBSD, see what the Penguin busted, and any can be the "master" at that time, pushing sources back to the others (but the Mac will be the ultimate repository, or it might be the FreeBSD server, etc).
Sounds to me like you are describing a git hosted software project, which you then clone and build where ever you need it. Each clone is its own repostory, which you can commit to and push back to / pull from the central repo (which will be, for instance, the github version).
