Comment 3 for bug 1087186

Here is how I think it should be implemented:
1) rhythmbox should use GApplication to implement a few GAction like "play".
2) rhythmbox should install a .service file to be dbus-activatable
3) rhythmbox should install a org.gnome.Rhythmbox.desktop file that tells the actions it implements. See http://standards.freedesktop.org/desktop-entry-spec/latest/ar01s07.html
4) Unity can then start rhythmbox and play by invoking the "play" action on rhythmbox, using g_desktop_app_info_launch_action().

Most of the work should be done upstream IMHO.