A luau sounds great! If I was getting married in Hawaii, that is what I would want to do. I think your guest will be really excited and have a great time. I am getting married in Key West while my cruiseship is in port. My ship is only there from 7 am to 2 pm, due to beach cleaning, we won't be having our ceremony until 11:30 am. We are planning to go to Margaritaville after for a few drinks and a small cake. We have been telling every since we planned the wedding, that there is no reception and they are fine with that. They have the option of staying home and looking at the pictures online.
Good luck and enjoy your Hawaiian wedding!