I'm still trying to implement this navigation structure in my app. I've got further with it with the help of Kevin's navigationGroup example, and using 'heavyweight' windows for the Android side of things.
To help other people out here's the code I have so far; it's easy to add this stuff to a new project so that you can have a play & see whether you can work out the issue.
Basically, the code below will work on Android, letting you use the back button to navigate back to the main menu after navigating down a path. But on the iPhone the main menu appears but pressing down to the next level gives you a navigation bar with a blank page underneath.
The issue may be related to this one that Kevin has just confirmed is a bug, but the workaround doesn't work for the code I have here.
Here's a link to the zipped up Resources folder so that anyone who wants to try this can get up and running with zero effort.