I guess hooking up some sort of mic and routing it to your headphone amp... ....provided that the person in the booth has the headphones on....
Talk back on the consoles that have them is good, but I suppose your thing (AW) doesnt have that...(?)
Well, I guess you simply can put in a mic on one of the mic inputs and mute it, and route it to the monitor output. Then unmute it when you want to talk. That will use of one if the mic inputs, though.