I doubt that will happen. If you dine earlier, you can come back and watch the fireworks, showing your receipt. They will not let you upstairs before a table is ready for you. You can certainly ask, but I have not read of people being let upstairs to watch fireworks before the time of their ADR. But anything can happen.
"I think you will be happy to hear that you can absolutely head up to California Grill before your reservation to view the fireworks. Just go to the California Grill check-in desk and let the Cast Member know that you have a 9:40 reservation but would like to view the fireworks from the observation deck."
This is our plan for 9/14 with a 9:50 pm reservation.