It's a cute top! I might use a piece of polyester batt as big as the heart, stitch in the ditch around the heart and trim away the batt along the stitching lines. I would then sandwich the quilt top with a regular 80/20 batt and quilt the background. This will cause the heart to further puff out for a trapunto effect and you could just do enough quilting in the heart to meet the batting requirements.