Not for android that I know of. As stated many car systems recognize iphone/ipods through the USB cable but that seems to be where it stops. Do you not have a USB port? I personally just put music on a USB drive and use that.
Also, why do you say your line out quality is that much better? The A2DP systems I've heard have sounded just like I was plugged in via the aux input. Maybe you should try a different media player on the phone to see if it helps. I personally recommend PowerAmp, as to me I have yet to hear anything better than the Direct Volume Control poweramp offers. Streaming via bluetooth gets you the same equalizer settings as what's applied to the headphone jack, so it sounded identical to me.