This topic was mentioned not to long ago but I cant find in what context.
There are a number of illustrations and photos that suggest some of the wagons where covered. Probably the most famous of these photos would be 1 of a wagon crossing the Tugela at the lower drift. Also in Lloyds paintings there are quite a number ilustrated. One of his paintings from the top of Shiyane looking through a telescope at isandlwana shows covered wagons abandoned on the saddle and one toppled into a donga, a number of them are covered.