WM has the pens/markers for fabric that are permanent. They did have them, anyway. I remember seeing them for around $4 each, and I think they had two tips, one finer than the other.
I'm not sure if those are what my Grandma uses or if she uses the Sharpies. Try Sharpies on a scrap first, and iron it, then throw it in the washer a few times ...see how it works out.