This isn't a bad setup. Create some bones that are children of the ribs on the front, lats on the back. Have them stretch to the hips on the front side and the glutes on the back side. DONE!
Well, okay, the breathing thing takes a bit of effort. Basically, model ribs into your character, and drive a smoothing constraint on her ribs area with a bone for breathing, and also have her ribcage be children of that same bone so the ribcage grows/shrinks. And then constrain the breathing bone's axes to prevent the ribcage from expanding in silly ways.