I am back from my trip. (BTW, it was with Overseas Adventure Travel. A good balance of cost and quality, IMO.)
In the end, I took two bodies, a 5D Mark II and a 5D Mark III. They share batteries/chargers, which was convenient. I brought a half-dozen batteries, but 3 would probably have been enough, even with no electricity on some of our stops.
I brought the 70-300mm, the Kenco extender, and the 24-70 f/4. (At the last minute, I decided to bring that one over the 24-105. It's smaller, newer, has better macro capabilities.) I was pretty happy with that combination, but if I had to do it over again, I might take the 16-36mm f/2.8 instead. I missed going really wide. Not really for landscape vistas, but for interiors and for stuff like the kopjes.
300mm plus Kenco extender was generally long enough. I didn't miss my 600mm f/4. I did miss the speed of my faster lenses. But considering weight and all, the 70-300mm was perfect.
Thanks for the advice, everyone. It was really helpful. Especially the tip about the filters. I did not need my rocket blower. I never used the circular polarizer. But those B+W filters were gold. Dust stuck to everything except those filters.