Difficult to meet all your requirements, but here are some suggestions that could help you out.
Close approximation: Bluetooth receiver with audio output connected to your Onkyo receiver. Then just play directly from your phone using the Google Music app. May require switching audio source in your Onkyo.
I'm not familiar with the interface of those apps and what remote apps are available that would control them without a TV being on. But to bypass the TV state you can get an HDMI splitter that funnels the audio to both your receiver and your TV. Check Monoprice for an HDMI splitter or switch, which model you'd want depends on how many other devices you'd want to hook up through it. But something like http://www.monoprice.com/Product?c_id=101&cp_id=10110&cs_id=1011002&p_id=5557&seq=1&format=2 has an audio out which you would connect to your Onkyo receiver and allow the music output to bypass the TV. It does further complicate the theater setup, of course. But ideally if you funnel all your external devices (cable box, streaming devices, etc.) through the switch and just connect it to your TV as little more than a monitor, then you just use the switcher to select your current audio/video device.
Using the splitter should get you halfway there, then you just need to see what device/app combination will allow you to control music with no screen on. Haven't tried that. I use Retune on Android to control my iTunes playback (which mirrors my Google library using Google's Music Manager) on my PC/Mac, which is connected to a TV and speakers. Apple makes an iTunes remote as well. These work without them being the frontmost program on the PC. Unfortunately I haven't gotten an Apple TV or any of those other devices to say what remote apps work to play music without them being in the front. If you go with the Bluetooth receiver method, you could also combine it with the remote app and stream audio from a computer. At least that way you can use multiple controlling phones and a single library. Should also work with Wifi audio receivers such as the Airport Express (and Apple TV, they both use Airplay).
There's also the Airplay method from your phone (there are several apps to provide Airplay on Android), which would interrupt whatever else is on the screen and play the music, but it's also playing the audio off your phone like the first suggestion, and would require an HDMI switch to avoid needing the TV on.
Best bet, then is probably HDMI switcher + iTunes + Music Manager + PC/Mac + Apple TV + Retune and Apple Remote apps. Not quite what you wanted and more complicated on the back end, but I think everything you want on the user interface end.