Halo just posted functions for getting the extents of a mesh, its a one time calculation/transformation on load if you dont want to fix it in the model files.
your other choice is to scale the y position when you scale the model:
new_ypos = terrain_y + ((old_ypos - terrain_y) * scale_factor)
your other choice is to scale the y position when you scale the model:
new_ypos = terrain_y + ((old_ypos - terrain_y) * scale_factor)