So I saw this and it got me thinking.
No, it has not been made yet. But, it would be thooper easy (except it's actually not) to DIY!
Instructions (alpha stage?):
1: Get thin case, w/ no covers for headphones/charing port
2: Put an extra layer on case
3: Get Thumb drive+USB OTG cable
a: Take casing off USB
b: Put m/board + other stuff in the extra layer of the case
-Optional: if you dont like fancy lights, remove it from the usb
c: attach Thumb drive USB to USB OTG
4: Stick the male part of the OTG in the charging hole of the original case (preferably solder it so it stays)
6: Profit! (literally)
I don't know, just thinking out loud.
Currently owned devices
Nexus 5 (Rooted, Cataclysm w/fauxkernel)
Nexus 7 (Rooted, Stock)
HTC One X