You can remain in your seats after the first showing of Fantasmic to watch the fireworks. Of course, you do not get the view right in front of the castle. However, we found it to be quite beautiful with the fireworks going off right over the Mark Twain. The perk of getting to keep the seats was definetly worth seeing the fireworks from a different viewing area. Also great to not have to camp out hours for a spot.
There is a podium set up across from the Haunted Mansion near the churro cart by the ROA. We were told to check in at 7:00. However, since seating is made according to the order you check in I arrived at about 6:30 to be sure to get front row seats. There was one man already ahead of me. The cms told us that they had never seen folks line up that early but I was sure glad we did because apparently everyone else decided to show up well ahead of time that night too. You only need to send one member of your party to check in and select your seats. Afterwards you can leave and return with your group about 20 minutes before show time.