If the cushion is already constructed, it may be too difficult to get it under the presser foot to stitch down. If it is not constructed, than pick a pretty stitch on your machine and go to town. If you do have to do it by hand, you could pick a fancy embroidery stitch and embroider the edge down. You could also do the invisible hand stitching you suggested. It is going to look very nice from what I can see!