http://www.getinpulse.com/apps/itunes/

This app displays a pause/play button, a next button and the contents of a non-pulseprotocol notification. This app comes with a host-side python app that connects to the iTunes and changes the song, or plays/pauses the current one based on your clicking

After loading iTunes to your watch, you can start the Python host-side and control your iTunes, all from your watch.

Would this work solely for iTunes, or can it adapt to an Android's music player?

If music control is exposed in the Android SDK, it should be trivial to make this app work with Android.

All of our example apps are open source so you can see exactly how they work.