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Wrap whole butts in foil and place in an insulated cooler with towels top and bottom...they will hold temps for hours in advance.
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This is the winning answer right here. Start the butts to leave yourself a ~4-hour buffer on time (this may also work well into your schedule so you don't have to start them in the middle of the night, start before you go to bed and check them when you wake up). When they are done, wrap in foil, line cooler with towels, put foil-wrapped butts in, and leave it shut. The butts will hold in the cooler for hours with all 6 of them keeping the others warm. I once did about 40 butts at one time, naturally, with that many, and of varying size, in varying locations in the smoker (big smoker) some were done hours before the others. I held the first-cooked butts in the cooler for ~8 hours until all the help arrived to pull, sauce, pack them. They were for a few LARGE weddings and the pulled pork got rave reviews, numerous people said it was the best pulled pork they'd ever had.
That being said: if you MUST cook in advance and reheat, IMHO the ONLY way to do it is pack in large vacuum bags and chill. When ready to heat, put the bag in a large pot of boiling water to warm the pork up (bag still sealed), this will prevent the pork from drying out. Once it is warm dump it in the pan and sauce it if you're using a finishing sauce etc. (don't douse in BBQ sauce, that is bad Pulled-Pork form).