If you get galvanized metal hot enough, it will give off some toxic stuff.
The temp required to do that is well over 400 degrees I think though, so I'd say galv. metal could be ok depending on where you're gonna put it in your smoker.
For example Wood smokehouses are popular, so that tells you the walls of a smoker don't always get real hot (and galv. metal shouldn't give you any trouble as a lining there)
If you line the heat source area with something like stove brick or just keep the galv stuff a foot or so from the heat source, that might do the trick.
Infra-red heat guns/sensors are pretty cheap these days, you could check out the temps inside your cabinet to see where the hot points are so you can avoid them in your design.
If you wanna test your metal and see what the deal is with galvanized, take a couple inches square of it and heat it with a torch. If I remember right, it will start turning color and give off something that looks like smoke and smells real bad. do this outside and don't breathe the smoke
