Battery Box for Bicycle to support PNA (Navi), Mobile, ... (devices that can get power via USB)

I have a Falk iBEX30 cross PNA (Navigation System) on my bicycle with a battery that lasts about 4 hours. Too less for longer tours, but it can be powered external via mini USB for example by a power bank or by the generator for the bikes headlights. The only problem with it is, that it goes into USB device mode if a common USB cable is used and shows just the USB symbol on the display. That doesn't help me much if I want to know my route.

One solution could be to modify the USB cable so that