Left-button popup menu with GNOME 2 panel applet?

Is there a simple & direct way to add a popup-menu for the *left* mouse 
button (in a default setup, i.e. "button 1") to a panel applet?

I can easily add a *right*-button menu by passing XML starting with
    <popup name="button3">
to panel_applet_setup_menu(), but if I change "button3" to "button1" in 
my defintion, the menu will *not* pop up when clicking button 1 (and the 
items won't appear on the button 3 menu.)

-- 
- Toralf