I just got tethering working using Bluetooth between the N9 and Nexus 7. I prefer Bluetooth tethering when I don't need a super-fast connection as it goes easier on the battery.

I installed an app on the Nexus 7 from Google Play called "BlueVPN [DUN Client]". I then paired the devices, started BlueVPN, and set the Access Point (3G) name in the settings. I could then link to the N9 and use my 3G connection.

I've also read that this works more reliably when wi-fi is turned off on the Nexus 7.