Why not a 21.6mm x 21.6mm sensor?

Vignetting isn't a problem. The GH2 doesn't have more vignetting when shooting 16:9 images than it has when shooting 4:3 images. The diagonal of the sensor area that's used is the same in both cases. A MAR sensor just takes full advantage of the 21.6mm image circle with all the supported aspect ratios. The part of the sensor that's outside the image circle isn't used.