You can always ask, but my experience has been they will tell you what times are available with the package for the restaurant you choose. Not all times will be available (they have a limited number of packages available-it looked to us like about 1/3 of the Fantasmic stadium was roped off for Fantasmic dinner package seats, but that's just an estimate.) And once they book enough dinner packages to fill those seats, that's it, no more packages so if you want to do this, book as early as possible.
When I reserved Hollywood & Vine in February for May 2008, I was told that 5:30 was the latest seatings they ever do for Fantasmic dinner package. In April, I reserved Mama Melrose for September FDP and was told the same thing.
Our seating was around 5:00 at the buffet. We were seated within 10 min. of checking in, and it took probably a little over an hour (just the two of us) to eat. Afterwards, we had a couple of hours to shop & to ride a couple of rides; we wandered around some before going to the special Fantasmic entrance to be seated around 8:20 or so. Was a pleasant evening, for sure and did not miss standing in line for more than an hour for general admission seats. (In May 2007, we missed Fantasmic because the night we were there, the line for the show started forming long before the gates opened...there were hundreds in line 90 minutes before it started! We weren't about to wait in line that long when we only had one day/night to be at that park.)