Take a look at the Location Plugin - https://wiki.fogproject.org/wiki/index.php?title=Plugins
It allows you to have a “master” FOG server that you could point all clients to (in terms of the FOG client installed on the computers), but have them image from the server at their building. If you have a computer at your office for repair, you could also switch its location while it’s there to image and then switch it back once it’s returned to the building. We use this plugin very successfully with 24 sites, although we only have two FOG servers total since our WAN links are all 1Gb or better.