The sensor in the camera, be it film or digital, does not move w.r.t. the lens when zooming. So zoom lenses have to be designed to focus the image at exactly the point (plane) where the sensor lies, regardless of zoom length.
So, yes, a tiny back-focus adjustment can and does work. That's how Canon themselves do it when shipping the lot back for service -- it's just an offset value that gets programmed into the lens and/or camera firmware.
The issue here is whether or not they continue to make this adjustment user-accessible or not.
Zoom lenses are designed to not change back focus when zooming/stopping down- the question is do they? Some older lenses were notorious for having "focus shift" when the aperture was stopped down. It's possible the same thing can happen with modern lens if quality control is an issue. Of course stopping down your aperture can mask many focus ills so it may not be an issue- especially with slower or wide angle lenses that have a lot of depth of field to work with.
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