For anyone interested… I’ve hacked a few fudge factors together because the scaling is not well-behaved. And it seems to work pretty well (the different coloured sky is deliberate)…
Until…
When the players is very close to the window, the scaling breaks down quite catastrophically.
I can’t see a workable fix for this. Cranking up the resolution on the render target “works” but hobbles performance. So, ultimately a dead-end. At least, for now.
I’ll use this technique for a mirror and a portal but only for the sake of completion, and out of curiosity.
A pity really, this method looked quite promising early on but let me down in the end. 