Control RH viewport background from within GH


I kept searching for a solution around the new and old forum and I don’t seem to find anything that doesn’t involve C/VB/Py/etc. scripting…
Basically what I am after is a way of integrating into a GH definition of a way of controlling the RH background. What I have in mind as an animation project requires a non-static background. I wish I could vary the colour, use a more complex gradient or, ideally, to import bitmaps (as frames of another animation).
Any ideas?