I have my Nexus 7 connected to my Bose head unit through a USB DAC which frees up my Bluetooth for OBD and also gives much higher quality audio connection. I mostly stream Pandora or Rhapsody from WiFi, but could just as easily save my music on my Nexus 7 or even just put it on a thumb drive and plug into USB Hub.
To describe in more detail, I have a OTG Y-cable coming off the Nexus 7 with one leg of the Y connected to the USB Hub output. The other leg connects to the 2A charging port on the USB hub. The USB DAC input connects to one of the hub inputs, and the USB DAC output connects to the Aux-In input of the head unit through a standard 3.5mm stereo cable. Since my Bose head unit did not include a rear Aux-In input, I patched one in which was very simple to do.
I have to point out that this configuration requires installing a custom ROM described
here. This enables the Nexus 7 to simultaneously charge and operate in Host mode for the USB hub. It also enables the USB audio.
Besides selling the tablet mounting bezel, I am planning to offer kits that provide this unique capability, and I'm currently working on the website which will provide step-by-step guides for completing the full installation and expect to have this open by July 4th.