Just to round this topic off, the Newertech dongle arrived today and, after only about 15 minutes messing around with the settings, I got it working fine with the Mac Pro. To answer a couple of questions that this thread raised:
IIIass - The early 2009 Mac Pro IS compatible with AirPlay. You have to allow CoreAudio to bypass the firewall, but once you've done that it works fine.
Johnb-one - I am using it for mixing (Logic 9, can't get on with X, hence still on Mavericks) songs. Like all good mixers, I check the mixes on different speakers, two of mine happen to be AirPlay. Before I was having to bounce songs from Logic to stereo AIFF, copy AIFF to USB dongle, copy AIFF from dongle to iMac, play via AirPlay from iMac. This was a lot of faffing about, so I wanted a solution to play songs direct from Logic on the Mac Pro to the AirPlay speakers. AirDrop doesn't work. but old-style file sharing does, so that's fine too.
The only down side is that there is chronic latency, around 2000ms, but that's not an issue as I'm just listening back to the mixes via AirPlay, not mixing 'live' as it were. But the fact that I can now do it direct from Logic is saving a huge amount of time and mucking about!