I wanted that too, when building terrain, but i found a workaround for that.
I used a grid texture on the terrain… you can calculate the needed grid size by synchronizing heightmap-size with texture-size. The result is very exact. So if you dont need the mesh to be transparent, it works pretty well that way.
Just apply the grid texture as the first terrain-texture. Make your Texture just a square with a border and set its size to 1 in the terrain inspector.
Look at the terrain docs if you dont know how to add textures to the terrain-object at all.