Do you know if you are going to be shooting the food at the establishment? Do you know if you are shooting in the kitchen or the dining room?
If you don't have one, I would invest in a de-shadow box.. You can get one with a pair of halogen lights for fairly cheap from B&H or Adorama. I got my girlfriend one for x-mas since she likes food photography and it does a good job lighting food.
http://www.adorama.com/VRDSB16.html?sea ... &item_no=2
Costs around $50 bucks, includes two lights and a blue/grey background sweep and most importantly folds up flat with a handle for portability and has pockets to store the lights. One advice though, do not leave the lights on if you are not taking photos. They get very hot!
Alternatively, you can build your own de-shadow box pretty cheaply from a cardboard box, some white tissue paper, white cloth for a sweep and some lights.
I took this picture with the aforementioned de-shadow box. I put a piece of white cardboard paper in it to create a white sweep for the background.
One halogen lamp to the left and my sb-600 triggered off camera above and to the right.
If neither of these options are open to you, or if you choose to shoot the food in a more authentic background of the restaurant then I suggest you bring your fastest lenses due to probably the dim lighting of the restaurant and the ability to control DoF and a tripod of course. If you can, make sure they get you the food as quick off the grill as they can because hot food usually makes for the best looking food. Especially for BBQ, you want to capture the juices sizzling from the brisket or the sauce cascading down the ribs.
I would suggest getting some food at home and practicing a bit with it so you can find what aperture settings you like best, what lenses you think you'll use the most and how to set up food for the best lighting.
Good luck and have fun. Make sure they get you a free dinner!
Oh, and make sure the plate is clean. BBQ can be a challenge because of this, so make sure you have some paper towels handy and a bowl of water.