We've treated this successfully with a brush (or very soft toothbrush) and olive oil- it gets under the scales and suffocates the mites. The correct treatment is Benzol Benzoate, which is also used on humans to treat scabies. Not all breeds of chicken shed their scales with the annual moult- Wyandottes, Leghorns, Marans and Buff Orpingtons don't. Our Orpington cock shed his at 8 years! Take care you don't tear the scales off. You can simply immerse the legs in olive oil of course. We treated one hen who had become increasingly aggressive- the effect was immediate and she returned to her former easy-going self, so it must be a very painful condition.
Please don't attempt WD40, which is one stupid remedy banded about a while back. Someone tried it and the chicken screamed in agony until it had all been washed out.