I just haft to add, *not trying to contradict Wolfy*... but I would never use a chain on a mare in an open pasture... if like you say she shys from lunge whips etc... then there is a good chance that she has also had a chain used on her improperly as well.... Under the chin = head up... sometimes waaayyyy up. If she bolts from the shock of a unexpected chain tug, then you could loose her in an open field dragging a line that could get tangled etc... not good. (not to mention I've seen several fingers get broken that way)...
Again... no Wolfy bashing, just my opinion...