I contacted Jack's Magic and I must say that these guys are quite knowledgeable about stains (even knew some chemistry, which surprised me). Anyway, for rust they recommend an oxidizer first (they use something that sounds similar to a non-chlorine shock like KMPS except higher in oxygen content) and then a stain remover that sounds like dry acid (possibly sodium bisulfate). I'll get some of their products and see what happens and let you know. I suspect that ascorbic acid just isn't strong enough by itself.

Richard