Robin, did you ever figure out what that odd thing on your chickens back side was?
My chickens are just now starting to lay again after an enmass first molt. Anyway yesterday I was around the coop all day and one of the times I was in the coop one of the girls had just laid a "soft" egg that broke on it's way out, the egg insides were in the nest and stuck to her tail fluff was the wet limp remains of the egg shell, but that got me to thinking about the pic you posted. If I had not been there and seen it and taken it off her, if it had dried there I think it would have looked very much like your pic.
How is she doing by the way?