I don't know the answer but I know someone who does. Ask in the <a href="http://GoodMillwork.com/crown-moldings">crown molding</a> post at Good Millwork and I'll make sure they get you an answer and add it here too.
The only thing I can recommend is caulking the uneven gaps. If you have a white ceiling, this would blend in fine. If you are painting the crown moulding, pre-prime before installation, then caulk the trim to ceiling gaps, then use a high quality, interior latex paint for a top coat. Should hide any imperfections enough for you to be satisfied.