Stephen--
Sounds like you're shooting for the full, sensory, WOW factor (which I wholeheartedly applaude!). There are a few potential problems but with a little foresight these can be mitigated. You don't want the butts so undercooked that you have no control of finish time once you get to your next destination (nor so undercooked that you foster food safety issues), nor do you want to interrupt the rendering process (you can, but I don't think I'd go there).
Plan an overnight so that the butts would be just done at the last possible moment before you have to leave (there is some flexibility here). (Shoot for 188-190 tops; if they hang in the mid-180s let them go for a while then pull them out--don't push them higher.) Wrap in foil and stick them in a cooler as Adam suggests. Disassemble your cooker and cart it, along with some fuel, your chimney, and some smokewood, to your next location. Bring something to fill the waterpan with, your sauce(s), knives, bowl, etc.
On arrival assemble your cooker. Light a chimney and when half lit dump it in the cooker and toss on a little wood. Put cold water in the pan. If there is a lot of time between then and the time the butts need to be 'done' don't add the butts yet. Close vents down so that the cooker chugs along at a low temp and the smoke wafts enticingly. Unwrap the butts partially and put them in when you have 30-45 min (tops) left to go. (Toss in another piece of wood if needed for effect.) If the butts are pullable when you unwrap them (they should be) keep the cooker temp very low. If not quite, raise the cooker temp, but in either case cook 30-45 min, re-wrap and rest back in the cooler. They will not need a lengthy rest before pulling.