Probably setting up a param to the .ini file like "AcceptAnonymous 1" and then if a user_id is not found (ad_verify_and_get_user_id ?) then assume the posting is anonymous.
I needed to do that to the classified ads too. I have a ads domain for jobs and I want to let anyone post to that domain. Same to the Neighbor-to-neighbor (I set up a category for CS students to post their opinions on professors eheheh).
