No active image found in material

Yep, I know that there is many threads with this exact problem. Though, as much as I find these answers, they has told that user should add image for every material. But what if I have no intention of baking textures on every material?
I have used Texture Atlas addon, merged all (or part of objects) into group, created material for this atlas and added Image Texture with image of this atlas.
Nevertheless, all materials from this object are transfered into texture atlas and, while baking, Cycles requires images or every material.
I could just remove all materials (except of this one with image for baking), but some of them add transparent effect and, thus, light will not get through solid objects.

How to cope with this?
scene3.blend (5.95 MB)