Thanks for the informative reply, Carl! And while I'm at it, thanks for GreenSock! I'm loving it so far!
Using your advice and a bit of experimentation, I was able to get this working great!
You can see it here! It seems that ScrollMagic is smart enough to pause the animation and hijack it itself as soon as it gets it's grubby mitts on the tween object. :3
The relevant code was:
var scene1 = new ScrollMagic.Scene({
triggerElement: "#parallax1"
});
scene1.setTween(new TweenMax($("#parallax1"), 1, {
backgroundPosition: "center 200px",
ease: Linear.easeNone
}));
scene1.addTo(controller);