Another tester just had success on copper stains that had turned black using muriatic acid with the tool I will link below...this is likely only a suitable approach for plaster...and would still require some metal sequestrant, i suspect...
Not especially cheap, but it seemed to work well for him:
Amazon.com: Purity Pool UAW Underwater Acid Wash Set: Patio, Lawn Garden
(Since the MA worked on it.)
I have been looking to get this kit but there doesn't appear to be anywhere outside the USA to get it
I have seen it on Ebay for about $140 when everywhere else is about $70 but they don't ship to the UK
Does any one have knowledge of a retailer over there that can or will ship here