I made a module to abstract the drawing of screen space quads. I tried to control it like a div, so the size can be set in pixels and percentages, and can be anchored from both top/left and bottom/right.
https://www.npmjs.com/package/three-screen-quad
Live:
http://dusanbosnjak.com/test/webGL/three-screen-quad/
Hope someone may find this useful, and would appreciate feedback. It works the way i wanted, but the code is a bit messy.