I would call and find out ahead of time. When we ate dinner there this past September, it seemed you had to have a reservation to even get up to the resturant. I did see some eating in front of the sushi bar, but do not know if you can just walk up or not. Again, you need to check in on a lower floor and then an elevator takes you up. Call first so you are not disappointed.
You can go to the Front Desk, on the 4th floor, and ask if there is a room at the bar, or if you can just go get a drink. Nine times out of Ten, it wouldn't be a problem (During Fireworks is that 1/10 time).