How to show media controls on lock screen when playing music?
I had this problem - to resolve it, play a track with Google Play and lock your tablet. You should notice the new lock screen with album art is now present.

Now, quit Google Play etc. and go back to using (the excellent) doubleTwist. You should find doubleTwist's "now playing" track appears on the lock screen just like Google Play.

I suspect Google Play starts a background service enabling this new feature. Hopefully an update to doubleTwist will do the same in future - but I'm just guessing.