Of course if you decide to rent, why not rent a 600mm with a good tripod? or a 500? I own the 70-200mm f/2.8 IS II and the 100-400MM L. I often think about selling the 100-400mm and picking up the 2x tc to use with the 70-200mm but I just have a hard time doing it, I love the 100-400mm almost as much as the 70-200mm, especially on my 5d MK III.
It's a hard choice but I've heard a lot of people on the forum say that the 70-200mm with the 2x III TC will give you just as sharp and beautiful images as the 100-400mm by itself... I already owned both, before I thought about the TC. Now I think about getting the 1.4 ver. III TC for my 100-400mm, especially when I think about tossing it on the 7D!
Difficult choices, but for a wonderful trip like the one your getting ready to take, I'm leaning with BWFishing and thinking renting would be a great option, I'd still rent at least a 500mm though rather than the 100-400mm...
Good luck, hope we get to see some of your images after you get back!