I've held one for 4-5 hours with no problem, but 8-9 sounds a little extreme. If it were me... I'd wrap it in foil, wrap that in plastic, wrap that in a towel, and throw it into my already warm cooler and hold it that way for as long as possible.
From there I would do one of the following:
1) Remove the towel and put the wrapped ball in my oven on the 'warm' setting and let the oven hold it for the remainder of the time.
2) Cut up the brisket, vacuum seal it, put in refrigerator, and then reheat while still sealed in a hot pot of water.
I'd prefer #1 unless I needed my oven for other stuff or if I didn't have an oven with a 'warm ' setting. Option 2 is the only way I'd want to reheat the brisket.