I have a global pref in my Keyboard pref pane that maps 'Zoom' to cmd-ctrl-z (iTunes default for zoom this window) and it works exactly the same as clicking the green gem plus or selecting Zoom from the menu.

Ah, and the apple falls from the tree.

Holding down option and clicking zoom will zoom all windows.

This is (apparently) implemented by having the zoom method (of the window) check the keyboard state and look for the option modifier key.

So binding zoom to a key equivalent which contains option, will invoke this “global” behavior.