I hate the old facilitation trick of using parking lots to close conversations. It's rare that anyone takes accountability for answering questions or concerns captured in a parking lot. What more often happens is that the point is forgotten - except by the person who brought it up in the first place, who could legitimately feel ignored and now has a reason to disengage from the effort. Parking lots can be places where good ideas and honest concerns go to die.
One way around this is to modify the concept of the parking lot slightly, and instead, use parking tickets. When an off-topic issue is raised, the participant or facilitator captures it on a sticky note, AND makes an exact copy of it. The facilitator then asks the group who will take responsibility for following up with the participant who raised the issue at another time. Once someone volunteers, they get a copy of the sticky. The other copy of the sticky goes on a "windshield" flipchart. This serves to capture the issue in the group memory, and to remind the volunteer of their obligation the next time the group convenes. If no one volunteers, the issue goes back to sponsor.