I went on the WAT shortly after it opened, so they may have changed policies on things. When I went on the Trek, you could only bring things attached to you or your safety vest. Your clothing is covered by the vest and straps go around your waist and upper legs. You can't access your pockets or bring a separate bag unless it was maybe a small lens case clipped to your vest. You would also have to convince the guides that there was zero possibility of dropping a lens cap while changing lenses. You don't have much stopping time on the trek either, so just having opportunity to change is unlikely. If you have an all around lens, I'd suggest using that. If not, serious zoom is still your friend. You get closer to some animals than on the safari but you still aren't eye-to-eye with lions.
Someone with more experience on a recent trip may have different information. I think our tour had a gentleman that brought 2 cameras so he could have two different lens options. It poured during our trek, so lens changing would have been that much worse on that trip. Oh, and your camera has to be strapped to you as well.

Enjoy!