I have an older Brahma hen who most probably has some chronic condition in her egg ducts that has caused her to quit laying and to have constant runny poo. I love her dearly, so she's honorably "retired". But her crusty rear does require some occasional maintenance to restore her fluff.
I prepare a wash tub with warm children's bubble bath, and while grasping her firmly, I hang her crusty butt over the tub and splash water until the crust softens. Then I gently wash it off with a wash cloth and towel dry. If it's cold, I throw in a free blow-dry, which most chickens adore, believe it or not! Even my boys get this spa treatment to "tune up" their rears, and they love getting blasted with warm air.
No rule against chickens looking their best, and I imagine getting that load off their rears has to make them feel better, too!