I played with everything under the sun. C4's native, Autonomic, Grace, Airplay, etc.

Use Sonos. With the EV app it works great natively within C4. You can tell your more savvy clients that they're using Sonos for their digital media and point them to the Sonos app. Name each player as a source, and then they have full access to radio, music library, and other music sources (including Songza). It's a 4 finger swipe to go back and forth as necessary.

I feel like I'll be murdered for even suggesting switching apps. You purists don't have to - you can just use Sonos within EV augmented C4 and call it a day. I think there's value in flipping to the Sonos app until C4 rewrites the iPod proxy or adds another to deal with media. Maybe it'll come in 2.4.

No storage is an easy problem. Add a NAS. 4 Sonos Connects and a 3TB mirrored/redundant NAS is cheaper than the MMS5A. Given my experience with the buffoons at Autonomic, I won't be looking to them for any solutions anytime soon. My love/hate relationship with Sonos will continue.