It would be easier to mold it in place in plaster than to make a curved wood crown moulding. There are two ways I know of to go about it. You can either cut the curve in solid wood and make the molding on a shaper, or buy already-made molding and cut it apart into bands in the plane that you want it to bend. Since the saw blade removes a thickness of material as dust, you need to cut the leaves of molding-veneer from two pieces of stock, making up for the wasteage. Then the molding-leaves are reassembled with glue over a caul (form) of the desired radius. When dry, the excess glue is scraped off, and the molding cleaned up for use. Plenty of chisel and gouge work will be necessary to blend your bent molding into the straight stock. The advantage of this laminated bend it that the grain is consistent throughout, unlike a curved molding run from a bandsawed blank. A laminated blank molded on a shaper is actually best, provided you have a shaper.
Flex-trim will make you just about anything you want- I know they do crown molding to go around, say, a column, so they'd probably be happy to do a vertical-plane curve.
The choice you make will be dependent on whether it must be stain-grade, paint grade, or the machinery and time available.
Casey

Thanks, Casey. I guess I would have to make a template of some sort if I wanted Flex-trim to do it.