I've been kind of thinking about that myself. I was thinking about the ole' PVC pipe bit for long arms, but this kind of limits the speed of access.
I was thinking clean it up good, oil it good, and stick a bunch of those Vapor Phase Inhibitor (VPI) tags in the PVC pipe with it. (I also toyed with the idea of leaving a BoreSnake in the greased barrel with the brass ("aglet") end hanging out the muzzle for a quick cleanout of the grease in the barrel. Just haul the rest of the BoreSnake through, jam a mag in, and you're ready. But I really didn't like that idea so well.)
I also "kited off" on the idea of using degreased fine steel wool in a permeable packet to take up oxygen if anything in the tube got damp.
But there's still the access problem. I guess the solution there would be to make sure you paid attention to your surroundings so you wouldn't need a firearm in quite such a mad rush.
I guess, if you could wrap it up in clean cloth and then really (I mean "hermetically") seal it up in plastic, that would be OK. I don't trust plastic being directly in contact with gun metal. ( Related point: I know for an actual fact I can fire a revolver while it is in a thin plastic sandwich bag. Doesn't work so good with autos.)
I was also thinking of using an ammo box for a handgun --same kind of preservative methods.
I would not do this with expensive firearms. Not just for theft loss, but for the possible corrosion problem.
Although theft is a concern, the tradeoff is needing one and not having it, versus someone stealing it or towing the whole vehicle away for parking tickets or something dumb like that.
One slightly more expensive alternative, given that you believe that shotguns are good defensive arms, is the Mossberg Marine kit.
http://personalsecurityzone.com/cgi.../PSZ/?Template=ProdDetail.htm&ProductID=27187
Then there's always Stainless Steel.