I've done a similar thing in my home-brew rules for the period.
Haven't got the references to hand but I'm sure I've read contemporary accounts by British officers praising the skill of the Zulus to skirmish, make use of cover during their advance and even the accuracy of their musketry.
There's also accounts of them anticipating British volley and artillery fire, dropping down as soon as they saw the infantry go to the 'present' (IIRC, the drill book of the period had dispensed with the supplementary order to 'fire'; the men were trained to fire automatically almost immediately thereafter) or when the artillerymen stepped back from the piece once loaded.
At Isandlwana the British seriously under-estimated the effect of their fire (severe though it was) as a result of this tactic.
The Victorian Wars forum is a great place to ask questions like this - some very well-informed gemtlemen there!