The Backdrop is appended to the <body> as a sibling of the <ion-nav-view>. That’s why the z-index alone doesn’t help you here.
I made this work by calling $document[0].find( 'body' ).append( element ), so the element is, like a popover or modal, positioned in the DOM after the Backdrop. Now you need to set the element’s z-index > 10 and violá.
Maybe not the most elegant way to do it… well I’m not a pro, yet.