muslin likes to shrink, as in it could shrink so badly that you can no longer clearly read the signed names. :(
Some people here like th Pigma pens, but they dry out easily if not properly cleanedand maintained (I was a draftsman for years and used them for work, they are not my friend).
If I was doing it I would send them a big square of muslin at least 6" square if not 8" and ask them to SIGN BIG. Then when I had the squares back I would take embroidery thread and stitch over the signatures setting them forever. Then you don't have to worry about bleeding or running, and if they sign big enough you don't even need to worry about prewashing.