I have looked at the Godot-Demos/viewport/2din3d demo and it helped with the basic idea for me. So I put a 3d Quad into my 3d world and will render 2d into that.
But... I linked the Quad to my camera so that is nice BUT... I am changing the camera's FOV as the game goes. That means the Quad changes size on the final screen display as the FOV changes. Which is not what I want for a HUD.
Should I change to doing 3din2d instead? That way the Quad goes away and has nothing to do with the camera. The HUD becomes an real overlay, if that is possible + I can figure it out some day.