Actually there are several photos of 744s with missing winglets on this website. I think there may even be a A330 with a missing winglet. I am all but certain that a winglet can MEL
'd. I think the last thread on this topic stated there is a slight yaw moment if the winglet is gone but it does not significantly alter the flying characteristics of the aircraft. If that thread was accurate, it seems like a little rudder trim will take care of the yaw moment.