I will post a pic tomorrow. We're pretty sure it's from the rooster because: A) This hen gets mounted a lot more frequently than the others, B) The feather loss started gradually, and C) None of the other hens have feather loss this bad.
She does not appear to be losing any more feathers b/c at this point the rooster's claws are on her bare skin when he mounts her.
The apron is off currently (she got it off about a week ago and we haven't put it back) but normally it's on full-time and had been for several months prior to when she slipped out of it. We plan to put it back on tonight. If nothing else, it should provide some protection from the sun.
I'll try coconut oil, good idea! Will look into bag balm, never heard of it.
Peck No More sounds a lot like Blu-Kote; we have some of that but I think it's pretty drying and her skin is already very dry. The other chickens are not pecking at her back so that's not a concern.
We're also going to try to trim our rooster's toenails per someone's recommendation in another thread.