If I understand correctly what you are asking look at this thread http://www.diychatroom.com/f14/crown-moulding-flat-ceiling-angled-wall-question-43318/. from a couple of days ago.
Hello, I'm in the same situation, I have a bay window with rounded inside corners and trying to find a solution for my 4 1/4" crown moulding. I thought of trying the bullnose method, but haven't gotten around to it yet.Yes, that's what I'm looking for. Inside corner, bullnose for crown. Or a good looking alternative.