You're right: canvas and image planes were not the same thing (otherwise, why to have both?).
At the same time, please note that "image planes" are marked as "removed" and (I'm copying/pasting) "Removed indicates that the feature or component has been removed without equivalent functionality."
I can only say that I realized Autodesk removed from Alias almost every features that were tipically designed for video production needs (and still there with no updates from the time of the old Power Animator software). Of course, occasionally some tools designed for video production were useful also for design needs, as in your scenario.