Good summary/review of the options Mike, thanks!Paul, you are touching on a lot of different topics and they probably need to be separated a bit.
There are 2 major ways to do distributed audio: The first method is to have centralized source equipment (music server, internet radio, CD changer, terrestrial or satellite tuner, etc.), and centralized amplifiers. These systems require both data cabling to remote controls (keypads) and audio cabling to the remote speakers. These are the NuVo and Russound type systems so often discussed here.
The second method is to have a centralized music repository and distribute the music digitally to source equipment and amplifiers located in each listening area. In these systems all cabling is local except the network connection and even that is often handled via WiFi. This is the method used by systems like Sonos and the Logitech Squeezeboxes.
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So I assume then that in your setup, each squeezebox has its own pair of locally powered speakers (the AudioEngine A5's?). At one point, I thought I had understood that you had centrally located amps which fed speakers in each room so I was wondering how you had resolved getting the line level audio back to the central location (i.e. amps).
In answer to your other questions:
A squeezebox system can display messages from a Home Automation system but it is not a Home Automation controller and is not the way to control other devices. Also this is not simple to implement because it is not directly supported by most Home Automation devices and requires you to work with the xAP protocol which might not be where you want to focus your efforts right now.
What led me to this question was my assumption that the weather and stock ticker were likely some types of plugins to the server. If the SqueezeServer plugin API can also intercept incoming events from the user's interaction with the squeezebox, I figured it would be possible to have the plugin interpret these events and pass them on to a home automation server... or as you say over xAP/xPL. This opens up a number of possibilities and might allow the squeezebox to act as a stand-alone interface. S-W-E-E-T!The weather and stock display features are plugins for the Squeezecenter application that acts as the music server for Squeezebox players and are not dependent on any Home Automation application or hardware.
Can anyone chime in here and confirm that the server plugins can do this type of event interception or are they limited to display updates only? I must admit, that everything I'm suggesting implies the server would have to be able to control quite a bit of the squeezebox's display, manage contexts (music mode vs. home automation controller vs. whatever else). Clearly not what it was designed for, but who knows.
However, as you pointed out, such a setup (assuming that I'd want the squeezebox speakers centrally powered) would require me to run the line level audio from the back of the squeezebox down to the amps in the wiring closet. Hmm... if the server really had full control of the squeezebox, I guess it could even theoretically control audio levels, feeds, etc. That would be a way sort out automation announcements... but I'm surely going overboard with its capabilities.