USB sync is not possible, due to limitations from Apple.
Your central sync database cannot be on an iOS device. You either need to have a Mac which all other Macs/devices can sync to, or a WebDAV server which all of them can see.
It sounds like the WebDAV server could be on your front network, or you could use Omni Sync Server if all your Macs can access the internet.
If this doesn't fully answer your question, please contact us: 800 315-6664 or +1 206-523-4152 or
omnifocus@omnigroup.com
(We try to keep up with the forums as we have time, but email and phone are our primary support channels.)