Yeah, I have a couple of Light Brahma's. I have to inspect their rear end every so often.
If the weather is warm or hot, you can hose off their rear end with a spray nozzle, then release them. Do this around noon time, the afternoon warmth and heat will dry them off before they go to roost for the night.
Otherwise you'll have to put the hen in a container of warm water and soak her rear end to loosen the feces out of the fluff and feathers while wearing disposable gloves.
Then pat dry her rear end with an old towel and if you wish, use a hair dryer to completely dry her off.
You can trim back the fluff with scissors so she have a clear "shot" to the ground, but dont trim back too much fluff or the urates in the feces can scald her skin presenting another problem.