I'd not bathe them, won't solve the problem and is super gross, but trim off the feathers with poop on them. Repeat as necessary until there's no more 'catcher feathers' on the path.
I would recommend trimming as well. A bath takes care of what is there now, but doesn't do much for anything going forward, unless you want to be routinely bathing chickens while it's freezing.
You can trim moderately, mostly concentrating on the area right under the vent (where I would go shorter, and then leave the area around longer to hide it a bit if it bothers you), and that should leave them with fluffy behinds, but lessen the amount of build up.
One of our Brahmas had a poop stuck that seemed like it wanted to build up, and I couldn't take it. Snipped a tiny bit and soaked the rest off with a rag dipped in hot water. Gross, but it worked. I never got her actual skin wet. Even though it was warm out, she wasn't having it when I tried to dip her butt.