Andorid 4.1 now supports Audio through USB! so in 6 months when we get it your problems will be solved
On a more serious note, I believe your Lexus is not treating the device connected to it through USB as a mass storage device. What I mean by this is that it doesn't read the file and process it and then play it. In the case of your iPod, the iPod is doing all the processing and just outputting the processed sound, through the line-out pin of its connector, which your car is then merely piping to the speakers. This is not much different than an aux cable connected to the headphone out jack, albeit a cleaner signal.
Bottom line is that this functionality has not yet been available for android, until 4.1 that is.
edit: that's one of the benefits of having a 30 pin connector, you got so many damn pins that you can use them for all kinds of things. USB is 4 pins (sometimes 5, but the 5th pin is just an ID pin and useless otherwise) so you need to be more clever with your implementation.