Hmm now I'm wondering about spraying their butts with Teflon!!!(only kidding! but it might work!)
I have a hen that loves to play in the flooded area around our irrigation box. She is always caked with mud on her feet and butt, she actually bathes like a dust bath in the mud!!( she thinks...
I have a few really fluffy butts that get poo balls forming back there, I take scissors and trim the balls off, and put a small amount of ACV in their water helps firm things up....