Is it a stain or a scratch through the porcelain surface? If it is the former you could try a mild oxidizing agent (like a dilute hydrogen peroxide solution) or mild reducing agent (e.g. Sodium thiosulphate solution- photographers "hypo") to oxidize or reduce the stain to something soluble that will rinse off... Disclaimer: this works for lab glassware stained with metal salts (usually iron (III) and permanganate - they're a bitch to clean off
) but I've never tried it on porcelain. Test it on an old basin first!