Interesting discussion. Best solution I have come up with so far (not exactly what is being asked, but close) :
I have a MacMini installed as media-center. The USB harddrives are actually connected to the TimeCapsule through a USB hub (so that TimeMachine can still run on my other machines without having the need for MacMini to be active).
On the MacMini I have set up the "log-in items" on my user account to include iTunes and each of the USB hard disks on the network. At least that way each time the MacMini starts up (e.g. after a power outage), all files are accessible and iTunes is running.
Using Caffeine (a little tool available on the AppStore), I make sure the MacMini never really goes to sleep.
Not ideal, but it does make sure that whenever I feel like playing music on my AppleTV, I don't need to first grab the keyboard of the MacMini to start that up ... it's always "online".
Hope this helps a little