I gasped when you suggested machine stitching across this! I think I'm seeing that the top was put together with hand stiches so my thinking is that you need to follow suit with hand quilting. Also, hand quilting was characteristic of the time the quilt was assembled. Do be careful though as some of the fabrics may be prone to being torn with the "stress" of the up and down motion of the needle. This is one place that larger stitches may be in order, reducing that stress on the fabrics--more supportive than decorative but still adding that extra dimension with the quilting design. Liked the idea of a motif inside the white spaces!