Is this a new Nexus 7?
If so you need a different kernel to allow OTG and charging at the same time.
As far as I know OTG charging is not included in that kernel.
Also, if you are powering a hub and other devices off your tablet you may find you can't do it all and may need some different options.
There are powered hubs available and some other cool stuff.
I will give you a link that is refering to a hacking platform for the Nexus 7 (2013) but there is much information in that thread about powering the device and some custom wiring etc to make it all work.
I think you will get all you want and more from this thread if you read it all. Also I hope you have the 32gig version... Although the 16gig version will work the 32gig version has more flexibility.
I have three ROMS loaded on my nexus and still have over 24gig available on the device.