Boy do I wish I could get my hands on this one! I've been sitting here studying it. I think your idea of SID is correct for the colored area. Then you have all that black. You have black diamonds on either side of the main large ones. I would quilt that celtic knot in those, in black. The sashings, I see, with a simple rope winding all the way through. I think I would also do the rope all the way around in the borders. Then you have the diamond centers to the braids themselves. I think I would just do curved lines around the interior. (we longarmers call them pumpkin seeds or orange peel) Then you have around your colored celtic knots. I think a tight stipple would set them off nicely. I'd do a little looser stipple in the triangles on either end of the braids.
Can't wait to see what you do with this beauty!