Stumbled across this looking for a similar solution, ended up creating this function:
var playBackwards = function(timeline) {
var retval = false;
retval ^= timeline.reversed();
if (timeline.yoyo()) {
retval ^= timeline.totalTime() / (timeline.duration() + timeline.repeatDelay()) % 2 > 1;
}
return !!retval;
};
Perhaps this should be brought into GSAP itself.