To ever so slightly quench people's thirst, I was doing some digging a while back and I came across the portfolio website of Dylan Cole.
https://dylan-cole-j5dx.squarespace.com/return-of-the-king
It has some stills of the matte painting work he did in their original resolution. Quite frankly they're beautiful.
Sadly he seems to have updated his website and has reduced the section on Return of the King.
He used to have a lot more examples, as well as a feature on all of them to click and cycle through the different elements. There was a great one where you could see the before and after of a Barad-dûr shot showing the original shot of the miniature