September is a tough month since the NoBo hikers are done with NH and are in Maine racing to the finish. If you can help up there the hikers would love it since they are sincerely in need of it. They are achy, tired, worried about finishing and there will be a lot of people to help. I like to base myself in Stratton to do the most good.
Southbound the hikers won't be accumulating much until the weather is getting cold. Try Bob Peoples at Kincora in Hampton, TN around November. The hikers will pile up at his place if anywhere. I've been there at Thanksgiving time and found them coming in 5 at a time. By that time there is snow occasionally and the hikers are more interested in hosteling it than staying outdoors every night. Bob is also a great guy and can answer any question you have about the trail. If you stay there a few days he'll rope you into doing some small amount of trailwork as well I'm sure.