Tough call. My first thought was a double wavy line through the center of the sashing -- or even a cord. But the other part of me feels like the piecing is so modern that simply echo quilting the blocks might be a very clean & striking option. Assuming your sashing finishes at 2", you could space the lines out evenly every 1/2" or maybe instead go with a triple line in the center with 1/4" in between the lines.