This is a set of udev rules, bash, and python scripts that allow you to pipe audio out of the USB port of a an Android device out to your DAC. It utilizes a little known part of Android that has exists in all devices Android 4.1 and up. Audio out of the Android device is 16bit 44.1Hz. Installation is easy and directions are on my github page.
With some further work, it is possible to send play/pause, stop, track forward/back commands to the android device as well. There is a script in the repository that shows how to do this. It would be cool to integrate this into the webgui long term. I would like to see if we could include this docking feature in future releases of Volumio. I think this is a good way to allow Android visitors to play music off their device since ios users already have airplay.
For the curious, this is code that I have been using on a BeagleBone Black for the last year to act as a car dock for my android cell phone. The beaglebone pipes the audio out of my phone, charges it, and receives steering wheel button presses via CAN and then sends the playback commands to my cell phone. So i am able to play/pause and skip tracks using the OEM steering wheel buttons in my BMW. The github page for that project is here.