The system music homescreen widget sends this intent for the built-in music player:
final ComponentName serviceName = new ComponentName(context,
MediaPlaybackService.class);
intent = new Intent(MediaPlaybackService.NEXT_ACTION);
intent.setComponent(serviceName);
pendingIntent = PendingIntent.getService(context,
0 /* no requestCode */, intent, 0 /* no flags */);
views.setOnClickPendingIntent(R.id.control_next, pendingIntent);
But it looks like this might take some hackery to implement outside packages in the music app itself because the MediaPlaybackService only accepts explicit Intents and isn't accessible from the outside. This thread seems to indicate it's possible with a bit of hackery, though.
But even then, as Roman said, not every music player will respect that Intent. You'll have to check with Spotify/Pandora/Last.fm themselves and see if they have any available intents to bind like that.