Don't know what kind of sewing machine you have, but if you have some fancy stitches, there might be a heart (or a flower etc.). I have a Viking and there's an outlined one and a satin stitch one. If you have a computerized machine, you may be able to automatically stop after one iteration of the design. If not, you can just sew very slowly and do just one.
Another idea would be to hand embroider a tiny "X", like a kiss, in the appropriate blocks.