I haven't ever tried to create a Minor Civ with basic BtS code, but I do know that the information is in there and would work MOSTLY like Barbarians. So you could do THAT with just XML. The main difference between a Minor and a Barbarian is that the Minor doesn't get free units regularly and can be wiped out by eliminating their last city (and they have a capital and whatnot).
As seZ stated, Fall Further has it set up so that there are multiple Barbarian Civs, and having been the one to set it up I can tell you it is a HUGE PITA. We still don't have every single bug eliminated from the system (the 2 new factions can't have cities, the citybar doesn't display properly if they settle them, fortunately we hadn't wanted them to have cities anyway. And a side-effect of the new civs is that we have some issues with loading a game or starting a new game unless you come from the desktop.
But yes, it does sound like Fall Further codebase might be fairly ideal for your mod. You can snag it now to start getting your hands dirty and figuring out what you can do and how to set it up, but I would think that releasing with our current bugs (mostly the loading one) wouldn't be a great idea since people might get frustrated with it fairly quickly (we're just lucky that people are already adicted to FF and keep coming back for more)