I don't know how possible it would be for a huge exchange like yuletide, but for smaller challenges I'd like to see an option on the sign-up form like "If your assigned writer/artist drops out, do you need a pinch hitter?" I know some people get upset if they don't receive a gift fic/art, but I'm not one of them; I am in fact upset only by the thought that the need to provide my gift is causing my writer/mod/pinch hitter trouble and stress.