[Baertram]

You need to define a top level contro(TLC) and add your other controls to it. The tlc is placed on the GuiRoot (parent of the tlc). And it's invisible/see through.
Set mouseenabled false at the tlc.

Set dimensions of the TLC = screen width and height.
Put one texture on the tlc and make it anchor topleft and bottom right (anchor fill) and use some see-through texture.dds, set desaturatio to the texture then as needed. That's what I'd try
If you already did that and it's not working I'm sorry, I do not know how the saturation actually works with the ESO controls, never tried it. So i'm just guessing here :-)

Using lua should do the same. You can either define all in xml or create the controls via lua coding. It's only a different Syntax.