Thanks for a solution Michael. Not what I would have expected to do, but the animation on an Xform modifier works.
So next, I was trying this to see if I could find a way of secondary scaling in the Z-direction after applying the rotation & animation. However, the animation is applied first, then any XYZ rotation or scaling, so scaling Z after the object is animated down gives a distorted parallelogram shape and not a rectangular box. I also found out that the animated geometry/ frame map solution does not work if the forest geometry is a single plane and not a thick object (I was trying a single plane in the FP, to then load a Shell modifier up the stack for thickness afterwards). A file is attached showing the erroneous scaling/ stretching that occurs. Being able to have user control over the order in which transforms are applied would solve most or all of this. A user could specify scaling in the Z direction first, then rotation on Z, then rotation on X, for example.
Then I tried manipulating the animation frame map to get other results than that spiral gradient. This uncovered a bug: if a perlin map is inserted into the animation map slot, and the perlin marble has pure white or pure black (or a very near color), the perlin marble crashes FP and 3dsmax. I've attached a demo file. In the material editor, there is a default perlin marble that you can plug in. If you swap this with one of the other perlin marbles, you'll crash 3dsmax.