I haven't looked at Safari's nibs, but you could possibly do this by adding an icon button and then setting its action to orderBack:.
You'd do this by placing the button, hitting cmd-1 to get the info panel, typing in the name of the button's image file, then cmd-dragging from the button to the item called First Responder. When you cmd-drag, a blue line will be drawn from the button to First Responder. The Info panel will change to the Connections panel. Connect the target to orderBack:.
I have no clue whether this will work, or even whether there's a nib with the faux toolbar stuff in it.
Good luck!