Say I download an mp3* on my Linux laptop. I want it to play on the housewide speakers, which are connected to a Mac. What can I do that doesn't require me to use VNC or ssh?
sherm pointed out that iTunes is not a RAOP server in any case, so looking into free software implementations of that isn't useful.
To respond to some of the other suggestions: even if I had network file sharing set up (which is messy given that our IPs are assigned with DHCP, the laptop goes on and off the network, etc. etc.) or set up a streaming server, I still need some way of telling the Mac "start playing". That's why I was looking at "push" mechanisms for audio.
Looking over the options I considered, writing an OS X backend for PulseAudio is looking more promising.
* Try this one. It recently got stuck in my head, even though I first heard it months ago.