(See final build pics below)
WARNING: This could fry your Touchpad if you set this up wrong.
5v Step Up Link: http://www.sparkfun.com/products/8290
I attached the 5v step up's (The step up is used to increases the battery's voltage from 3.7v to 5v) in directly to the battery, the step up's out to a USB extension cable and then the extension cable to the USB OTG adapter. Works great.
It's a little ugly right now, but really all I have to do is move the 5v step up to the SIM card slot (which in most Touchpads is not used) and attach a female micro USB port (a standard USB port is too large for the SIM slot) to it and then modify a Y cable with a male micro USB port.
The only downside of using this step up is it only supports 300mA, not the standard 500mA, so some devices may not work properly. Keyboards, flash drives and mice should have no issues.
Update: Final Pretty version
I cleaned it up and made it pretty, here are some build pics.
I had to sand the micro USB PCB to fit it in the SIM slot. Sparkfun part number: 9614
Side shot to show you about the width you need to sand it to so it fits in the SIM slot snuggly.
Placement of the 5v Step Up.
Placement of the micro USB PCB.
Side shot of the SIM Slot with the micro USB in it.
My modified male micro USB Y cable connected. The Y cable is just a Charge only micro USB cable attached to a USB extension cable.
Edited by ften, 16 May 2012 - 05:40 AM.