After reviewing dozens of available USB devices, guitar inputs, and obscure gadgets, I have given up the quest for a ready-made device that will satisfy my desire for a USB hub that charges the iPad, from batteries, while it hosts other USB devices. Currently, there is no such product. But one crafty, Dremmel-wielding hacker has created that very thing.
Untergeek tore apart an Alesis IO Dock and inserted a USB hub internally between the iPad and the IO Dock. It is almost as simple as it sounds if you know your way around soldering, hot glueing and warranty voiding. Here is a video on how to DIY it.