Z-Order of Nifty-GUI when used with default viewport

Hi guys,
looks like I’m unfortunately running into same problem as described here some years ago: Rendering problem with Nifty and addititonal processor - #9 by user2685

In short:
I need to run a shader as a post processor over the final image (3d scene + nifty gui) which still doesn’t seem to work when using nifty in a separate viewport.

A possible solution for me would be: to use nifty in the same default viewport, but then nifty is drawn behind the 3d-scene. So, is there a way to control the z-order of the nifty gui at all?

By the way: this solution should be much faster because the final fullscreen shader step is just running once and not twice (when using 2 separate viewports), shouldn’t it?

Any suggestions from the cracks?