Howdy Pete

Haven't been to northern Thailand but I reckon it will be a very cool trip!
I have used my 400 from atop an Elephant though It was quite manageable but I was in the fairly open scrub near the Zambezi in Zimbabwe not in a jungle. I would be opting for the fastest lens I have. So unless your 400 is a f2.8 (mine isn't ) i reckon it stays home. If you want to keep weight in general down, I would go for the 70-200 L 2.8 IS and a good wide angle (preferably the 16-35L 2.8 but since I don't have that it would be the 17-40L). Having said that you got a 300 2.8 for your Botswana trip so that would be tempting. 300 on a non full frame DSLR gives a nice reach for any monkies and birds up in the trees.
I look forward to your trip report!