I don't know if the fall is a good time or not. But last fall I made a large number of grafts and set them in a cool green house where the temperature at night was often only 4 or 5 degrees F above freezing in mid winter. I checked them frequently and though the scions stayed green nothing happened. They were grafted on the PT seedlings. When spring finally rolled around they started to grow. So I may have done better waiting until the Spring. Since you will keep them warmer, and maybe not onto PT root stock your grafts may not wait until Spring to grow.
