Does GarageBand iOS 1.2 support AirPlay?
It's a Treasure Hunt!

On the iPad: 

Click the wrench icon to open the Settings dialog. One of the settings is AirPlay - Bluetooth. Set that to On, and the usual audio output selector will appear in the toolbar.

On the iPhone:

Click the gear icon to open an untitled two-sheet settings dialog. Select the Song sheet. The AirPlay - Bluetooth selector is there.