Any way to set pivot point of a simple plane (Unity's default one) at runtime?

I am using this mode:
camera.transparencySortMode = TransparencySortMode.Orthographic;
It works and fixes a lot of z-ordering problems.
However it creates a few new z-ordering problems, like this large wave that is supposed to appear behind the hut and bridges, but does not… Well in fact, only the upper part of the wave have this problem; the bottom part has correct z-ordering (I mean that all gameobjects lower than the middle of the wave’s Y axis are correctly being displayed OVER the wave, like intended).

I googled and learned that z-ordering is using the pivot points to determine the transparency/z-ordering render queue. Obviously, the default plane mesh in Unity has its pivot point centered in both X and Y axises. After even more search, I found that unless I do my own plane in a 3D modelling software (I am expert at 3DSMax, this is not a problem) and export it to Unity, I have no control in the Unity Editor for modifying the pivot point. I found SetPivot (http://wiki.unity3d.com/index.php/SetPivot), an unofficial Editor script. I installed it, but it seems like it doesn’t work with Unity default planes… I am moving the X/Y/Z values from -1 to +1 but it seems to have no effect on anything.

Is it because I am not using the SetPivot tool like intended? Does it support planes, or only cubes and meshes? Is there another way I could define the pivot point without having to go in 3DSMax, create a plane, set correct pivot point, export, import in Unity, reassign everywhere, etc.?

For those who are more visual, a picture worths a thousand words:

You shouldn’t really be using the default Unity plane since it has far too many polygons for this purpose. Create a 2-triangle plane in a 3D app or use CreatePlane.