A brisket that large wrapped in foil and a couple of layers of old towels and stuffed into a cooler will easily stay safely hot (above 140F) for quite a few hours. You could time your cook to be done by 6am and that would give you a little extra cook time if necessary, and if it's done by 6 it'll still be fine at lunchtime.I'm planning to serve a 15 lb. brisket for lunch this Saturday.
I was going to start around 6 PM Friday, hoping it would finish by 10 AM the next morning at the latest.
Would it be fine to rest in a cooler for a few hours?