You could notch a few of your knee-wall 2x4's so they half-straddle the joists, then nail/screw/bolt them to the joist. This should be plenty strong. You would only need 1 or 2 of them for the run of the knee-wall. The rest could just nail to the bottom sill plate 2x4.
This is how the knee-walls were attached in my kitchen, but I tore 'em out! They were very sturdy.