Laura, my Mom does counted cross stitch, and she made a quilt from 50 of her cross stitched State Flower squares a few years ago. A major issue with the embroidery would be if the floss is color fast. If it ran, that would be awful! I imagine that Retayne and a color catcher sheet might help, but I would be afraid to try without testing first. A textile that old (well, it's younger than me
) is going to be quite delicate. My Mom uses a plastic basin, with a little bit of Ivory liquid dishwashing soap and cool water, and just lets her pieces sit in it for a few minutes. Rinse thoroughly with cool water, by letting it soak a few minutes, and then she lays it on a big towel to let it dry. If it's not really dirty, maybe skip the washing and do what I've seen recommended, vacuum the dust off the surface- cover the hose with a couple of layers of cheesecloth, or they used to recommend pantyhose, to keep the force of the vacuum low. They said don't touch the surface but go about 1/2 inch above the surface.
Doesn't Cindy Needham have a book about using embroidered textiles in her quilts? I remember there was a show with her, what number was it? I would believe what she says more than my not-so-experienced advice.
Kathy