I'm afraid that can't be done with Forest Pack at present. Although we can place objects on vertices, there's not a way to limit the geometry chosen by the spline. On the other hand, this would be quite easy to do with RailClone and as long as you disable slicing and bending for the segments shouldn't cause significant problems with memory usage. If you do use RailClone, I would suggest using the Selector operator set to Material ID mode and using that to select which trees are chosen.
I hope that helps,