Do the riders all come through in one bunch?? Are you counting on feeding 400 or so every hour for 12 hours? I have found that at big events like this people tend to come in big rushes at meal time. Most people do not want pulled pork or brisket for breakfast---so that narrows down the practical serving time also.
Five dollars for a sandwich and coleslaw seems very inexpensive to me, especially if you are going to share 5% orf the profit. Go to a McDonald's and see what you can get for $5.00---do not short change your own food. The only advantage to charging $5.00 per plate is that counting back change is easy. If you bump your price to $6.00 make sure that you have about $600 dollars in one dollar bills at close hand--Six dollar pricing will cause you to use a pile of one dollar bills for change.
If I was serving pulled pork and brisket ---I would have most of it cooked and pulled ahead of time and use my portable steam table to keep it warm. When I do large groups I try to cook something with a shorter cook time, and doesn't need pulling such as tri-tip or Top Block, and maybe chicken leg and thigh quarters.
I also use a lower end sandwich to help create profit--such as a hot link, bratwurst, polish sausage etc. These sandwiches often have a higher percentage profit---they cook very fast--They are not labor intensive because they don't need pulling---they do not take up much room in the smoker--and they serve quickly and easy. If you are helping the bar---sell hot links----hot links increase beer sales.
It is my opinion that slicing tri-tip or top block is less labor intensive than pulling pork--however the meat cost is higher. If you have friends and family that work cheap---then labor costs won't kill you. I have been told also that tri-tip is hard to find east of the Rockies.