I'm so not looking forward to fixing any of this, to be honest, but I guess it has to be done.
Unfortunately that won't work anyway I think. I don't know the exact details right now, but I'm pretty sure it will fail because the tiles are rendered to a specific area and anything that's outside of that won't be rendered at all. So basically the zoom shader is just post-processing the final image. I might be wrong, actually, I'll have to take a look.