In thinking about this and me always wanting a finished looking back. Could you not fold, iron and cut the corner and then place a piece of matching fabric inside, faceup and then re-fold the borders down and sew. You would have the mitered corners, and a finished looking back of your napkin. You would catch the fabric also soit would stay in place.... You could even place another coordinating fabric there and have a nice touch...
Hmmmm, now the mind is going crazy. Thanks ladies. LOL