[fjordaan]

That sounds like a very elegant solution, Joel. My first guess was to have a new submenu group in the Zoom menu, so you'd have:
View: Zoom: All canvases: Zoom to Actual Size / Fit in Window

But I think I prefer your solution.

I'm not sure it'd make sense for all Zoom operations to affect multiple selected canvases. What would "Zoom in" do? Zoom them all in from their current setting, or zoom them all to the level of the currently-viewed canvas? Ditto for Zoom to Selection.