I would use Matilda's Own bamboo. My longarm studio carries it, and it seems perfect for your quilt - lightweight and white, but opaque, and it has incredible drape after you wash it. I used it one day when I forgot to bring my own batting, and it's now at the top of my list, especially when the quilt has a white background.
The biggest problem is that it is hard to find in North America (it's made in Australia), but if you stumble across some I would suggest giving it a try.