Warning: Plain cooking oil sorted my hen's leg mites in a week
My barred rock had those rough, crusty legs from mites. I coated them with regular cooking oil each day after letting the kids out for school. The gunk sloughed off and her legs smoothed right out. Cheap, simple, and it saved a vet call.
That part about the gunk sloughing off is what made me think of something. The oil might work by smothering the mites, but it also traps heat against the skin. On a hot day, that could actually cook the skin and cause a burn or blister. I've seen similar things with other animals. Maybe try something like petroleum jelly instead. It still smothers them but won't heat up as much in the sun. Just a thought, because a bad burn on top of mites would be awful.