4.3 on the new Nexus 7 does not appear to pick up any ad-hoc networks out of the box.

Considering how quickly my N9's battery wipes when using wi-fi tether, I'm finding bluetooth tethering via BlueVPN in the play store to be a decent solution for now.

The main issues are that Google's applications aren't expecting an internet connection via bluetooth so you can't download applications from the play store or use some of their other apps; so for that reason I'm going to try to root and enable ad-hoc when I get a replacement device this coming week. But you can at least have some kind of tethering functionality through bluetooth in the meantime.