If you have the 2012 version of the Nexus 7, someone compiled a kernel that supports multiple USB devices to be connected to the tablet at once. This won't work on the 2013 version since the developer hasn't ported it.
http://mehrvarz.github.io …-power-management-nexus7/
Basically, this would allow you to connect a hard drive/pendrive and the SD card to your tablet at the same time. I'm unsure if you need an external power source for proper functionality but the Y shape OTG cable will assist in that.
Assuming you add a Y-shape OTG cable, a small external power source and then a small usb hub that connects your card reader and hard drive at the same time, your workflow then becomes:
1) take pics & fill card as normal
2) optional - i can view them on the tablet via wifi & eos app, rate/cull them whilst the sd card remains in the 6d
3) remove sd card from 6d
4) put sd card into card reader
5) put otg into the tablet
6) copy files from sd card onto the hard drive/pendrive directly
Solution still seems a little messy, there should be a simpler way to accomplish what you want.
Alternatively, DSLR controller might let you transfer your images directly to the tablet over wifi. I use the app and it works great on my device, but I don't have any wi-fi cameras to test this for you with. I do know that I have been able to transfer images to my tablet directly from the camera via an OTG connection in the app.
https://play.google.com ….chainfire.dslrcontroller
