Hey Malcolm,
Great question!
You may add additional webinars onto an existing schedule so that registrants may select the next available session in line.
GoToWebinar also offers follow-up emails to both Attendees and Registrants not in attendance. You may customize these to a small degree with your own messaging. Ref =
http://support.citrixonline.com/en_US/webinar/all_files/G2W020004 There is a redirect feature in Registration Settings that you may be interested in, though it does not fit your exact needs here. This feature is customized per webinar, and can redirect registrants to a site of your choosing after they RSVP.