As Adam mentioned, it's at Seafire Inn. That is the nearest restaurant to the front entrance, but not near enough. Guests who are only attending the luau still go through the front gate. As near as I can tell, those guests are simply released back into the park with everyone else when the show is over, so...yeah....
I can't swear to that, though. Maybe there's more to it that I haven't observed.
Well I can possibly be wrong but the I think the Luau starts just before park closing so by time the Luau is done, the park is closed so they can only go out the front gate (no rides)