My amp is hooked up the G4 in my room. My main machine is usually a wireless laptop. Of late I've been using a program called iHam on iRye to control the iTunes on the G4 from the laptop so the music will play through the amp.
iTunes should have a little pop-up the same as the airport express one that allows you to select which wireless enabled device you want the sound to come from. Then you could browse the remote library over rendez-vous and hear the music from the good speakers...
Should I submit this as feedback? Or am I the only one in the world that would ever want a feature like this?