Please don't sign up for the exchange unless you want to be a full participant -- signing up at the form puts you in the pool of people the software will match, and that would wind up a problem for the person who got matched on "this is not a real request."

We'll have a post during the signup period to let people know how to sign up to pinch hit; if you don't want to make requests but you'd like a chance to write for somebody, that's your best option.