Some Product design with USB 5V always powered , but for mobile device, this cost too much static current
Only For some AC powered product ,like Desktop , Communication system, and Automotive , 5V power is always on , some times on some products
While Your idea is sound like mobile phone battery box, right ?
As I know , BBB otg 5V power is connect to Power 5V
So I guess You can buy some mobile Phone battery box , and a mini-B usb cable will be enough