There was mention of SP-Offline mod tools down the line.
Was a while ago, discussed as a far-distant effort, mixed in with discussion on would happen if the servers were eventually turned off (the server code would be released so we can host our own).Can you find the source for that - it would be great if true, but I can't see it happening to the extent that you suggest would make a LAN game possible in the sense that the OP was asking.