Brentex, cement board is primarily composed of Portland cement with some fiberglass fibers and binders, no wood is involved at all. As far as for using steel studs for the frame, first off they may not be galvanized as they will never be exposed to the elements, secondly even if they were galvanized, as long as temperatures didn't reach the melting point for zinc, the galvanization would not wear off, if you season your smokehouse before use, this will coat it to minimize any such leaching. And even then, the minimal amount of zinc that would possibly leach off would be much less than what is consumed in zinc vitamin supplements. Even if the galvanization is cadmium based (with temperature extremes needed to melt), you should not have any issues. Also I would imagine that you would be applying the cement board to the inside of the smoker to cover the studs, so that alone will protect the studs from any issues. As far as exterior finishes, nearly anything you do should not affect the inside of the smoke chamber, so if you have cultured stone you like or need to use, feel free to apply. To affirm my recommendations, I've been involved in smoking and meat curing with my family and elders for 30 plus years and have been working in construction and concrete for nearly 20 years.