Yes, one can dine at Mamma Melroses, Hollywood & Vine and I think one other restaurant. THe waiter will give you a coupon that you use to get into Fantasmic through the back entrance and then you will enter from the far right (looking at the stage). The reserved area is to the right of the stage (looking at the stage).
You have to make ressies well in advance of your visit and you must dine at a specific time. We did it back in late January for a Friday evening (value season) so the benefit was minimal since Fantasmic was not even full for that evening. Such may not be the case during other weeks, but I know that even though it was value season the dinner reservations packages for Fantasmic were sold out when we called Mamma Melroses, but since we were a duce, they fit us in.
If you have a family with 4+ people and small kids, the dinner package is the way to go, otherwise you will have dine real early and get to Fantasmic real early to get a seat.