Adding an animation stream is nothing...

As I've said a million times before, it's not rendering in the background of the level, it's rendering onto thye level geometry, so the depth info is already there.

It's simply like changing the skin of the entity to match a background image

You can use whatever method you want for the BG image. I'm rendering a high quality version of the level, and then saving it to a TGA. Then the level is set to low quality and draws the old image.

It would be possible to input the depth I suppose. Such as writing the depth factor to the alpha channel. But then the case of rendering the depth properly becomes a problem...