As I understand it, at Centane, Rainforth's G Company, acting in a skirmishing role,drew the attackers onto the bulk of Upcher's force who were in "well-concealed" trenches. However, it was the "excellence" of the 24th's markmanship that ultimately won the day with the Gaikas/Galekas only getting to within 400 yards of the firing line.
I would suggest at Isandlwana, there was no need to draw the Zulus on ! Also, the ground at Isandlwana is essentially shale - almost impossible to dig trenches ! The firing line at Isandlwana performed as well as it had done at Centane, until it was ultimately outflanked. Trenches would have made no difference.