Another possibility from left field: Mirrors! You could model a plane to sit above the terrain, and a smaller, angled mirror to sit in front of the camera, periscope style, and set as a child of the camera in both position and orientation. The smaller mirror would reflect the image of the terrain reflected in the top mirror. A telephoto/wide angle effect might even be achieveable by making the top mirror slightly concave or convex. Of course, you would have to use the ART raytracer for accurate reflections, and there would be some challenges with lining everything up and making the angled mirror appear properly square in the final render, but it might work! Just a thought.