We just did this the other night. If you don't mind watching the fireworks from the ROA (and it's a pretty good spot to watch the show since there are water screens with projections), find a spot along the river where you have a good view of the screens. After the fireworks are over, you can get in line for your viewing area or do something else in the general area, then get in line. The second F! show is usually not as crowded as the first. If you want to watch fireworks from Main Street or the Hub, it will take you longer to get to the ROA since Frontierland is the only way to enter NOS after the show. So give yourself some extra time if you want to watch from there.