Actually, since this is part of iOS8, this already works with Set List Maker and BandHelper on the latest iOS devices. But you still need an additional app (e.g. "Midimittr") to establish the connections. It would be great to have this in your apps because I don't want to be dependent on 3rd party background apps.
For example, if you look into GarageBand in the settings of a song you find "Bluetooth MIDI devices" that allows you to directly connect one or more devices without the need for any other app. On the market there are already several, e.g. iRig BlueBoard, the Quiccosound MI.1 (already discussed in another topic in this forum) or the new line of Bluetooth controllers by Livid Instruments.