This is all so interesting! Great question! Since I’m new I’m sure this is wrong, but it’s just what popped into my head. A theory if you will 😉
If a layer is 2d and there are 3d layers on top of that layer, would the 2d layer always remain behind since there is no Z value for it’s position? You could use that for the “overwhelmingly large” backdrop that never seems to move in space. Or if you find it does move out of frame when you move the camera, you could parent it’s attributes to the camera attributes so it appears to stay at infinite distance. @.o
Or maybe render all the close camera moves seperately with a blank space, then with layer blending/alpha trickery drop an AVI background behind all that.
Hope this tickled your brain a little…? 🙂