I'm going to stick with my idea and give it a try when I get home.

My plan is to create my level lik I normally would, but using models and no blocks for the level. Make all models as high qality as possible, high-res textures, parallax oclusion mapping, whatever... but only render all that for one frame of every scene, then save that image as a TGA. Then all materials wil switch to the "special shader", and the last scene will be drawn on all models. The textures of the model will switch to a small 8x8 blank tex. I'll have all the quality I can get without the slow-down. I'll have an interactive 3D world as well...

We'll see how it goes...