How long can my smoked brisket stay in holding?
Well, I might as well answer my own question.

After 4 hours holding inside of a beer cooler, wrapped in towels, the internal temperature of my brisket went from 200 down to 146. I have read that 140 is the danger zone for bacteria growth. So I turned on my oven to 200 and placed the brisket (still wrapped in foil) in for the remaining two hours. By the time we were ready to eat, it came out in the 150s. While it's not an ideal situation to be in, this worked for me. The brisket still tasted amazing, it was a huge hit with everyone, and quite literally people ate every last crumb of brisket they could find.

After initially posting this question, I continued to search. It seems that one method people use is to heat up a brick in the oven and stick in the cooler with the brisket to try and keep the temps up. I didn't have a brick available. I'm sure I could have use a rock instead, but I didn't attempt it.