Port already is making a cel shadow shader, but a complete cel shader is impossible because we don't have access to the scene buffer or whatever you want to call it.
Looking at the recent soft shadows, shadow mods are becoming more advanced with every mod, even if they are gpu choking.
These would look awesome. I wonder if wire frame shaders are possible to be made in blockland.
I don't think you can disable rendering of bricks with shaders.If you could, that would be a really easy method of cheating.
I don't really mean disabling rendering, but rendered bricks with a wireframe included.