I would use any leftover t-shirt fabric to make a practice piece first -- as similar to the quilt as possible with interfaced t-shirt fabric, same batting, black backing. I would want to make sure that I could FMQ and not have white thread dots showing through on the back and/or black dots on the front. Getting a balanced stitch can be done, especially with a batting as thick as WnN, but when you FMQ the balance of the stitching changes back and forth as you sew. Also you want to make sure that your needle can handle the thicknesses.