Probably too much for the engine, but having dust hang in the air, for example, as a vehicle drives along it kicks up sand and such, and some of the dust stays in the air for a set period, slowly diffusing and settling - same for anything that throws up dust. I just remember playing Steel Panthers WaW on the African campaigns, and as your tanks engaged each other the air would become saturated with dust, lowering visibility as the battle progressed.
Other than that, maybe putting in smoke artillery rounds that can be called in, since with the wide open, cover free environments of the desert infantry need a way to cover their movements without getting picked off by riflemen at 800 m.