I just tried a 64G microSDxc card plugged into an adapter, plugged into a USB card reader and hooked up to my N7 via an OTG cable.  With the Nexus Media Importer app loaded, I was able to access pictures, music and video stored on the card.  I've used some smaller cards and USB thumb drives with The Media Importer app before but the fact that it can access this 64G card amazes me.

I tried this 64G card in the microSD card slot of my Droid Razr phone and my Samsung Galaxy Tab 2 7.0 and neither could read it.