Hopefully simultaneous charging + OTG w/ external HD will be a possibility.
Basically you want all three of these together!
http://www.amazon.co.uk/female-Micro-male-Cable-Adapter/dp/B005GI2VMG
http://www.amazon.co.uk/Griffin-Sma...desktop/dp/B00264FS5K/ref=tag_stp_s2_edpp_url
http://www.focalprice.com/CA0352B/U...tm_medium=GM_UK&utm_campaign=CS_GM_UK_CA0352B x 2
Which doesnt seem to exist!