it can prolly be done with a battery powered pico board but the enclosure would be a challenge if you didn't have a 3d printer or a hobby box n dremel or something
so yea a simple board....slim power bank...slim qwerty board....and lots of insulation to avoid shorting out something maybe
then again a power bank taped to a qwerty keyboard type phone cover would be easier....as the the other stuff you can prolly just use a universal case thats big enough for the extra thickness
My basic idea would be a universal mount on top with something of a locking drawer system to pull out the keyboard/controller and the battery at the top.
And is it just my phone or do most phones not function with the average USB keyboard/flash drive.
I have an "OTG" cable, it works fine for flash drives/keyboards on my 7" tablet, but not on my phone for whatever reason, I ain't never had an android phone before.
Apparently mine doesn't support that stuff, it's an LG Optimus Fuel from tracfone.