Definitely go for a respirator or some sort of lower half as opposed to a full gas mask, for many reasons. Most, If not all, gas masks aren't ansi rated, and they fog horribly. A respirator and a solid pair of goggles will keep you safe, functional and still keeping up with the look.
My buddy actually was putting together a similar setup. He found a sling pack at Walmart that was damn near spot on. I actually got one and use it almost daily. I picked mine up for ~$35. Would be perfect for holding a tank/reg and all sorts of other goodies. It even has molle on it!
But aside from that, I always loved the idea, and once I was actually out testing my buddy's HPA gun (with tank in pack) and a decent coat on, it just felt awesome.
And to be honest, that sort of setup is what I would love to run at an open world/post apocalyptic game, like Revelations out in Cali.
If I ever take the time to gather the gear together, I might take some pics.
