I just read that you can show up early to watch the fireworks before you eat, or you can return to the restaurant, show the elevator people your receipt from earlier in the evening, and go up to watch the fireworks. So anyone with a ressie for that night can watch fireworks!
From the official Disney restaurant page:
Magic Kingdom Fireworks Spectacular
California Grill offers one of the best fireworks dining experiences at Walt Disney World Resort.
Lights dim as the music is piped into the restaurant—immersing Guests in the magic of the night.
Dining before the fireworks? Bring your receipt to the check-in desk on the 2nd floor for reentry to California Grill. Then, enjoy the show either from California Grill Lounge or the outside observation deck.
Guests with a dining reservation after the fireworks are also invited to watch the show from the lounge or observation deck. Please note that fireworks may be cancelled due to inclement weather and are subject to change.