I'm doing something similar, although a more complex mosaic with maybe 50 extrudes, so doing the math for the offsets would become cumbersome at best. I think I could avoid that by using the attribute mapping tool to map the texture on each object to the same point.
Here's my confusion: in the sample picture above (with the camera across 5 boxes) the image is on one face, and stretched across the other faces. This is okay, but I can't figure out how to determine which face will get the image ad which will be stretched. In auto-aligned planar, it appears correctly, but can't be edited with the attribute mapping tool. If I turn off auto-aligned, the image goes to a different plane. I'm sure there's a rule, but I can't figure it out.