Amazingly I just posted this same answer to another thread about portable pens....My chickens come running if I have yogurt or a treat. It would be easy to get them in if yours were trained to come for treats. If you put them in the portable pen an hour or so before dusk they will go right back...
Beautiful coop and run. Nice job. Check this thread out if you are thinking of putting anything but chickens and water on your lawn. I really like AlienChick's use of the PVC for the mobile pen. I would need to add a top though to keep the hawks from turning it into a death chamber...