In my searches for a way to have multi zone audio i've come across these devices by NetStream., the SpeakerLinX SL220
They seem to be trade only, but if we could control them as an audio zone then all you would need to do is add speakers, unlike the squeezeboxes which need an amplifier as well.
These SpeakerLinX SL20 could be a really good choice for in ceiling speaker solutions without adding the need for full media directors, or extra kit in your rack with the CORE.
From reading the pdf files about it, they seem to have a web interface and use flash. Not found much else about them, but if these were supported i could see them being really useful.