OK, first point. There's absolutely no way that you should be doing this. Way too big and you are way too inexperienced at it.
Second point. Even though the organizer has said otherwise, there probably are some legal/permitting issues here.
Third point. As someone else stated, even if there aren't any legal/permitting issues, there's still some "safe handling" guidelines that have to be followed.
With all that said, there's absolutely no way I'd try to smoke and pull butts on site. Again as Michael pointed out, there's too much else going on. Start smoking and pulling the butts a week before the event. Immediately bag the pulled pork, crash cool it then put in a freezer. Move from the freezer to the fridge a day or so before the event so the pork can defrost. The night before the event, put the bags of pork in a cooler and ice it down. At the event, fire up every smoker that you can. Get the temp up to about 300 if possible. Dump the pork from the bags into disposable aluminum half pans. Spend the extra money and get the lids for them as well. Fill the pans to level, spritz liberally with a "finishing sauce" or some apple juice. Stack as many pans as you can into your smokers and give them at least an hour or so to reheat. If you have access to electricity, "roaster pans/ovens" would be ideal. Once heated, you could take the pans of pork out and dump them into the roasters to hold/serve.
If you don't have roasters, get some of the disposable chafing pan wire racks and some full sized disposable aluminum pans to go in them. Add water into the full pans, put one lit sterno can under each one (be sure to get the 6 hour sternos, not the 2 hour ones.) When the half pans have reheated sufficiently, pull them from the smoker and put 2 in each of the chafing pan setups. Then, reload the smokers with more half pans of pork. Get refillable squeeze bottles and a gallon or two of BBQ sauce. You can decide if you want to add the sauce yourself (when making their sandwhich, ask if they want naked or sauced) or just set them out and let people do it themselves. Better cost control if you do it, but takes less time if you let the customer do it.
You'll definitely want to buy a box of nitrile gloves, maybe even a twin pack from Sams. You might also want to figure out some method of portion control, other than just using pair of tongs. Get a scale from Walmart and practice at home. Make a pan of pulled pork, figure out how many ounces you want to put on a sandwich, put that much on a scale, then find some container that holds exactly that amount when full. Process would then be, get bun, open and put on plate, use tongs to fill container with the pulled pork, then turn container over onto the bottom bun.
OK, point number 4(or 6, or 7 or whatever). Would be really great if you had at least one or two other people to help you with this as even streamlined to some degree like this, you'll probably need it. There's just so much to do.
Last point, again, nothing personal, but you probably shouldn't be doing this. At least not thinking that you're going to be serving hundreds and hundreds and hundreds. Now, if you just want to smoke 4 or 5 butts and sell 50 - 70 sandwiches, that's a different story. I'd still smoke and pull before I got there though.