I'm not much help here, but thought I could share my experience with you:
My 4 BO girls picked ALL their beautiful downy butt feathers out last February. Literally over night. And they must have eaten them because their coop didn't look like the aftermath of a massive pillow fight. They have been bare monkey-butt chickens since then. It breaks my heart. I've checked over and over again for mites and lice, and even tho I found nothing, I dusted them and their coop anyways. They eat free choice crumbles and oyster shell and greens from the garden or my kitchen scraps at least every other day. They also get a handful of BOSS for treats once a day. They free range several days a week in our suburban backyard. For several months, I fed them kitten chow (with 40% protein), but it didn't affect any new feather growth.
They have 12 sq ft per bird inside the coop, so space isn't a problem. The only thing I can think of is that when it happened, it was REALLY cold and I was feeding them cracked corn as a treat at night to keep them warm during the overnight hours. Maybe I gave them too much (I don't really think so, but it's a theory) and threw off their nutritional balance. Boy oh boy did I worry about them for the rest of the winter -- it's awfully cold here in MN to have a bare booty. Regardless, it has become a habit and now whenever they get new feathers, they pluck them out. It drives me mad.
If they are bloody, I'd give them a squirt of blu-kote. I found that it seems to help raw, irritated skin and will mask any bloody feathers.
Anyways, a long story short, two of my girls are FINALLY NOW starting to regrow some feathers now that the weather is beginning to change. Keeping my fingers crossed that they'll ALL leave these new feathers alone and let them fill in. This year I'm going to be much more mindful of the corn that I feed them.