Found a decent solution I think -- I wrongly presumed that the tween
TweenMax.to('id', 1, {rotation: 360, repeat: -1})
would continue to increase the rotation beyond 360. Since it doesn't, I can simply do this on mouseleave:
TweenMax.to('id', 0.5, {rotation: 0})
Thanks anyway
PS -- I'm absolutely in love with GSAP!!