Actually, after a bit more experimenting, I've discovered this doesn't quite work... if I jump around in the timeline (ie. seek to a previous step), the loop will still be playing, when it shouldn't be. What I really need is to be able to set a boolean "playing" property of the loop from w/in Greensock (which would control whether the timeline was playing), rather than triggering it w/ a function call, so that when I jump to specific labels, that property is set correctly.
I'm having the same problem incorporating sound into my animations - when I move backwards in time, the sound still p