The Osprey "Rome's Enemies 5 - the Desert Frontier" has some information. Personally, I don't think Gripping Beast Arabs are a lot of use for pre-islamic berbers - except perhaps some of the heads. I think using Pictish cavalry, plus a few late Romans is a good idea. A few traditional Numidians might also be a good addition, especially for light infantry. Gripping Beast used to have a small range of desert tribesmen, which if I recall correctly would have been perfect. Not sure what happened to the range. It's not on their website, and I can't find anything with Google. But for infantry I might consider using the Perry plastic Mahdist box.