I think so. Most small trees only root down 12 inches anyways. A larger concern will be water, since containers dry out fast, and japanese maples seem to respirate a lot in the heat. Make sure it stays watered. The other issue is winter hardiness, depending on your zone, anything in a container should be considered to be at least 1 zone cooler than what you have in the ground. So if you're zone 6, that plant is now in zone 5, etc.