I’ve held briskets plenty of times in coolers with no trouble at all. Put a couple of extra fire bricks on your pit. Wrap those in towels and put them on the bottom of your cooler. Stuff in the brisket, and then fill the rest of the space with towels. It’ll keep temperature for many hours.
That said, it will continue to cook at any safe holding temperature, and it will get overly tender. It’ll still be mighty good, but it will kinda fall apart.
One thing you can do is to take it off early and let it coast while holding. It’s a late for it, now, but you also could have cooked it beforehand, vac-packed and frozen it, and then reheated.
The final thing you can do (and this is what I recommend) is to just do what you do and let the cards fall where they may. If you go through everything you need to do to cook a 22 lb. brisket and folks want to give you grief because it’s tender for holding it so long, well, they are the problem–not you.