I've been trying to set up a Fort mode game through a private xbox live game, but when I get to the scheme selection screen, it says that it's not available for "this type of networked game." So is the fort mode only playable through a local match or am I doing something wrong (the obvious answer).

It's a bug. Team 17 knows about it, and says it will be fixed. In the meantime, you can just make your own fort scheme and play a private match with it.