Well--in those days, in merrie Olde England and other places as I understand it, tugging at one's forelock was done by...people who considered themselves servants, as a sign of submission (and other things, mayhap). So if you have a person (and conceivably a horse though the control is far less certain) by the forelock or a horse by the fetlock, to me that indicates that you have control over the grabbee. I don't recall reading/noticing this phrase in L.W. either, but I'd think it was a safe guess to say that the character meant that she wasn't going to let time get away from her.