Just to revive an old topic with a kinda working solution that suits me.
I've been wanting to make use of my old Scandyna amp Dock that was collecting dust. What I wanted was to either airplay songs from my iPhone or Synology diskstation that sits on my home lan. Unfortunately the Synology dock doesn't come with an audio in lead (the V dock version does but they are like rocking horse ****).
I ended up buying a female dock cable from cablivejive called the dockboss+
http://www.cablejive.com/products/dockBossplus.html. I ordered it from the US and it arrived here in the UK a week later.
Although intended to allow other non apple mp3 players and phones to be connected to apple docks for playing music and charging (via the attached micro usb cable) it can also work in an airplay setup.
I have my Scandyna Amp Dock connected to two Scandyna micro pod speakers (sound quality is awesome). I've then connected the dockBoss+ adapter to the docking station and the 3.5m headphone jack to my airport express audio out.
Using this setup I can stream music directly to the speakers from my Synology device or iPhone via the amp dock. I can control the audio playback via the Synology Audio station iPhone app (or web gui) meaning that the airplay stream is coming from the synology device and not the iPhone.
It works really well for me as I can choose an stream audio from the synology device or iPhone wether it be music or the radio.
Obviously a device that would replace the airport express and dockBoss+ would be great but can't see it being developed by Apple.