I am using the UIAppearance protocol to set the background image of UINavigationBar objects throughout my app.
[[UINavigationBar appearance] setBackgroundIma
The Mail Composer view is run in a different process under iOS 6 and cannot be tampered with directly (since the view is essentially inside another app). You cannot customize what it shows, it's the same for the Twitter & Facebook views.
Here is a more detailed description of remote view controllers: http://oleb.net/blog/2012/10/remote-view-controllers-in-ios-6/