Change status bar applications' default icon

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http://unix.stackexchange.com – in fvwm, you can change the default icon of any window using Style <regex> Icon <icon_name.xpm>. In other words, I want to over ride the default icon set for an application without having to hack at system files. How can I achieve the same effect in awesome? In particular, how can I make sure that the application icon in the status bar is the one I set and not the default? (HowTos)