Haven't heard of any problems along the Kwakwatsi, though the shepherds certainly use the valley, and rustlers using the Bell Traverse / Mlambonja Pass also pass along this way. I'd say it's fine to camp in the valley taking the standard precautions listed
here.
If possible I would camp as far up the valley as possible, near the summit of Mlambonja Butress - away from the main trails. Another good alternative is Easter Cave. Though the shepherds know about it, it's harder for them to steal stuff than from a tent. In January this cave is likely to be wet though.
Camping between Castle Buttress and Cleft is the best place in the area I reckon, unless you opt for Roland's Cave. Enjoy!