Detailed resources for planning backcountry/primitive tours?
Are there ever!

I'm totally evangelizing for the adventure cycling association in my answer here, but check this out: Adventure Cycling Route Network. They have a lot of road tours but they do mountain tours as well.

You want the extreme of extreme backcountry mountain bike tours? How does 4418km of trail riding along the continental divide sound? Check out the Tour Divide, It follows the ACA's Great Divide Route.

Other than that take a closer look at the trail systems in National Parks, there are  trails that permit cycling, and some that are long enough to bikepack on for a couple days. At least there are lots where I'm from, which is near Glacier National Park.