I'm currently looking at sorting something out fro myself.
A device I am interested in the the Verbatum Mediashare Wireless http://www.verbatim.com/prod/accessories/wireless-streaming/mediashare-wireless/
this plus an OTG cable and a portable HDD will do what you (and I) want - when used with a table or smartphone.
Connect HDD to the phone via OTG
Connect Camera to Mediashare via USB
Use the phone to shuffle images from camera to HDD
However, I'm not sure this will all work - I'm still trying to find reviews, etc. which confirm it one way or another.