SEA-HND is a long flight to not have a seat assignment on (if it is the long segment).

On narrowbodies, there are almost always two or more bulkhead Comfort+ seats that are blocked for gate assignment. They should give these seats to you before they give them out as upgrades. As other posters have pointed out, the standby list also contains confirmed passengers without a seat assignment. That said, I would ask at check-in and at the gate as soon as you get through security (or as soon as it opens).