I admit defeat.
You cannot retarget a re-topoed plane to the original plane level zero. And you cannot retarget a plane to any other plane with different original topology. Also, a retoped does not become a zero level, so you cannot smooth and target back to the retopo, because the retopo is not at zero, and the target mesh must be at zero in order to create a UV.
Also, in any case at all, both planes must have UVs and both must mach at level zero which becomes impossible when you either re-topo in mud or change topology in maya.
I tried this in a number of ways, including using maya planes and mud planes with the related UVs.
At each point you are prevented from even trying to make a v map by mud. Therefore J was right, I have to change the space time continuum before making mud work the way I want.
Of course you can have holes in MUD, just have to import an object modeled with holes. But there is no way to make a vector of it * for use in TG as a terrain vector*. The best you can get is a cup form, or a tube with one end closed.
And yes, the no hole thing was stated a number of times in the big thread as well as here. But now I fully understand why. Which sucks, Just feels strange when you cant do something in 3D.
Anyway, I can do everything I thought. I just cant make a proper UV vector of it. So no *real* holes then.
@ulco, I'll just post less.
