How dark are the colors used in the quilt top? If you don't think they will fade I'd hand wash just the corners in cold water (you'll need help) and use a mild detergent and rinse well and hang in the shade on a clothesline. You can then press lightly or steam that section when dry. I would probably do one corner first just to make sure everything is okay. Is it just the bare corners, binding or a large section?