I live in Brooklyn, New York. How to protect for the winter a Japanese Dwarf maple in a container.
Most trees and shrubs in containers are hardy to about two Hardiness Zones warmer than when growing directly in the ground. Japanese maples are hardy to Zone 5, so you should be OK in Brooklyn, which is Zone 7. However, it would help to move the container (if possible) to a protected location away from severe winds. Otherwise, you could put straw or other insulating materials around the container to help prevent it from freezing so readily.