I don't think this feature exists yet.
I DJ with my mac and my iPhone. but i always wanted to use the iPhone as a second stereo output. for Cue, Effects and other stuff.

The main idea is to connect the iDevice to the computer with a usb cable. and transfer the audio trough the cable and the 3.5mm plug.
this way you don't need a router for wifi, you have 0 latency, you charge your iDevice at the same time. and everything is super stable! (Dj's tend to use the most stable equipment they can find).

i don't think there is an API for this feature in ios. but maybe in the future it will be available.