"As far as I'm aware, padx is an option of pack(), as opposed to an option of messageBox() itself."

Just to clarify that:

• -padx (and -pady) are standard widget options: see Tk::options.
• Those options are also used by a number of geometry managers, e.g. Tk::pack, Tk::grid and Tk::form.
• Tk::messageBox does not have -padx (or -pady) options.