I would not use the press and seal. It is a real pain to remove the tiny pieces left behind from the stitching and notorious for allowing your marking tool to transfer to the thread even permanent ink that has already dried. If you don't feel comfortable marking the quilt with a water soluble pen made specifically for this purpose (like mark-b-gone) then do what Cindy60545 suggested and use a water soluble stabilizer and make sure you also mark that with a water soluble pen. Gina Perkes recommends using the water soluble stabilizer in one of her books as well. The stabilizer will not stick to the surface so you will have to pin it in place.
Another option is to cut the shape out of freezer paper and iron it on to the quilt top and quilt around it. Remove the freezer paper and go back in to quilt detail.