We host hundreds of events per year and would like to redirect all registration pages to a generic "this event has passed" landing page once the event is over. Is there a way to put the redirect rules in place as a flow step in one of our smart campaigns?
In a Flow step? No, there's no interaction between the Admin area and Flows.
However, if you always store the expiration date in a Program-level {{my.token}} and include standard JS in every page to redirect if it's past that date, then that accomplishes the same aim (there are several existing threads about this).