What I would do is render each layer or plane as a separate image. Also do an mask render for each plane. Then you can either composit the images as separate layers in Photoshop (and blur each plane as needed) or you could reimport the images onto planes in Bryce. Use the mask render to make the duplicated sky and background transparent). Then set up your camera to see all the planes (maybe do some foreground lighting). Then render. Also, do a distance render to then do some final blurring in photoshop. This sounds interesting. I might give it a try too. =-)