I've not ridden that particular ride, but I might offer a couple solutions. For one...go fast. Seriously, the longer you drag out these dry sections, the more water you need, mostly as it relates to adding days to the endeavor. I've had to turn a two day section in a one day section just to reduce the need for water for meals alone.
Secondly, I see very few options for carrying more than 7 liters of water on a bike. For these types of rides, I use a frame bag as it allows me to put a three liter flexible water reservoir in the frame bag at a low point on the bike. That makes for better balance. The other three go on my back in a hydration pack. I can then sometimes find a home for another liter on my bars, frame bag or pack. Anything more than that.....B.O.B. trailer. At that point things start to suck.
I suppose you could go for a rack and then stash another few liters, but then you're talking about massive weight on the bike.
