I've never had to store my rifles in such an environment but I see no flaw to how you intend to do it. I also would coat them with Birchwood Casey Sheath and a gun sock would not hurt.. The thing with temperature change and humidity in your caseis, all of it happens over a course of time and not to the degree that could happen in other places. The condensation on metal comes when you bring a rifle in from the cold to the heat, or a warm house. In your case, as the room warms, so does the gunmetal. I do not think condensation will be a problem.

What I do (learned this trick from an old gunsmith)and you might want to.. get yourself a pair of cheap 100% cotton gloves, and check the rifles while wearing the gloves. This stops the salt off your fingers touching the metal. Then store the gloves with the rifles, in a plastic or paper bag. That way they do not collect dust and dirt. Some people actually will oil the fingers and palm of the gloves but I find mine pick off enough from the guns over times. I wipe the rifles off while wearing the gloves, and if I feel the need to inspect them, I wear the gloves. It saves inspecting them for the finger prints. Check your rifles about every two months in detail with a bore scope and all, and you should be fine.