I'm sorry, but I don't know how to put a button or any link to the script where it was in previous versions of nautilus. I can only use it from the panel. If you want it on the left, near nautilus's location bar, you could add a panel on the left of the screenand then put the launcher there... if you think it's worth it.
Regarding the persistence of the setting, I've tested it and what gconftool does remains that way even after a reboot. So the proposed launcher is for now the best approach to the button I know about... Maybe someone modifies and compiles nautilus again in some time...
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