to leave the ceiling sag and work around the sag with the molding to be completely compatable with the sag. Use the compass method and scribe the sag into the molding and take that away from the molding by a router, planer, or wood rasp. You know what I am talking about??? like fitting a kitchen counter top to a bowed wall?
Let me know your thinking. Its rather hard to diget with out seeing the radical ness of the project.