See the following AppleScript for the popular (and free) Mac OS X keyboard/mouse shortcut program QuickSilver that will get you a keyboard shortcut for the green zoom button. When you install that and use it with Quicksilver, you'll be able to customize your keyboard shortcut.
tell application "System Events"
if UI elements enabled then
set FrontApplication to (get name of every process whose frontmost is true) as string
tell process FrontApplication
click button 2 of window 1
--button 2 is the green "zoom" button for all applications
--window 1 is always the frontmost window.
tell application "System Preferences"
set current pane to pane "com.apple.preference.universalaccess"
display dialog "UI element scripting is not enabled. Check 'Enable access for assistive devices'"