I've read on the decorating MB thread that sharpie wears off (especially when up against a sweaty wrist on the inside of the MB) but that it also smears when coated with polish, so that might be something to test at home while there is still a chance to come up with an alternate plan. Maybe someone here has already done it and it worked? If it works that would be a wonderfully simple idea.
I'm a fan of the dog tag on the shoelace idea. My high school students made one as a joke for my baby shower gift. Well we actually used it as PP suggested, so it turned out to be a useful joke, lol.