Set active sub-header button-bar when using back button

Hello,
I’m trying to set outline “button-outline” on button-bar when it’s not in an active state… everything is working fine except the back button “ion-nav-back-button” which is not part of button-bar system and used across all views…I’m just wondering what is the technique you use to allow the back button to modify the button-bar state also?

Please see the link: http://codepen.io/anon/pen/MYebzy

Thanks,