Having had a look, I couldn't see any easy way to do that with the Web API. But I think it would be awesome to allow voting for adding or removing songs to the que.
What about some Applescript to trigger key presses on the machine on the network running Spotify: mute, play/pause, next/prev etc
Yea, actually... That could work. Or using the (Albeit buggy) Spotify web client, to trigger button presses. That would then work really well with the other APIs.