I use the string trick all the time but i use the corner of the closest eye, not the nose. Some of my portraits have the eye in focus but the nose and ear are out of focus. I love the look of an extremely shallow DOF.
Another trick if you have the movements is to us a a generous amount of front tilt to give yo just a horizontal line that is in focus then do the same with rear swing to give a vertical line in focus. The overall effect is a spot in the center in focus and the rest of the image is out of focus.