The following should work for you.

Connect iphone to BMW using y-cable

In I-drive select USB and make sure iPod music begins to play as normal

Now, hold down the iphone power button until you see the "slide to power off" screen. Release power button

Now press and hold the home button for several seconds. The main iPhone home screen will show and iPod music will stop playing.

You will now be free to launch any app and play audio through the BMW audio system.

Note, if you change audio sources in idrive and return to USB the iPod will return playing. If you select next or previous track on the center console or steering wheel control the iPod will return playing.

If the iPhone is also connected to the BMW audio system via Bluetooth, and the iPhone receives a call, when the call ends, the iPod will return playing.