I looked at the problem and figured I could just hack the tree shadows (see attached image). Here's how:
1. Turn off fake shadows, just let the raytracer in 3DS Max create the shadows.
2. Modify the camera parameters of the forest object. Make the single-plane tree billboards follow the camera.
3. Create a copy of the forest object. Leave it at the same position in the scene as the original forest object.
4. Modify the camera parameters of the new forest object. Clear out the camera settings and make the tree billboards follow the light, not the camera.
5. In 3DS Max, modify the general properties of the original forest object. Turn off "cast shadows". Then modify the general properties of forest object #2 and turn off "visible to camera".
6. This works great, and you get nice ray traced shadows no matter how the light source is oriented. But of course you need to create forest object #2 late in the design process since it won't automatically update if you change the tree layout...