one-piece OTG adapter works
I can personally confirm that this adapter works correctly as OTG on a Nexus 7. It is sold on ebay by a member called "soonwell-digital" in China.
I can't post the link, being a noob. Just search for soonwell-digital's store, then search within that store for OTG. They only carry this one kind of one-piece OTG adapter.
It also happens to work perfectly with my Samsung/Google Galaxy Nexus. I am running an SNES USB gamepad (soldered myself with the RetroKit from retrousb.com) through this OTG adapter into the phone running Snesoid. Snesoid does require that you customize the gamepad mapping for the buttons, so those of you who try this might get a little apprehensive if you try to play a game without first creating and saving an input profile.
The only bad thing about this OTG adapter is that because it is a one-piece unit, anything you plug into it just sticks that much further out the bottom of the Nexus 7. This could be a deal-breaker for some of you who use the Nexus 7 in portrait mode. It's not a problem for me, since I play Snesoid in landscape mode, and the OTG adapter sticks out to the right.
Hope this helps somebody. This thread helped me get my old-school gaming on--thanks guys!