Difficult to understand, what exactly you are asking.. If all you need to do, if you want all vegetation to stay below the height of the wall, surely you would just use the scale transform. If your vegetation model, is higher then the wall, you just scale it down, so that it's upper Z scale value is at or below the height of the wall.
Perhaps you could have rendered a simple 3D view, showing the terrain and the wall, then mark that up to show what it is you want to achieve