LOL. You don't think a metal, fireproof safe will degrade the signal at the other side of the house?
You will get no signal at all inside the safe.
Not to mention the issues of power and heat. If that drive (and Airport express) are sitting inside there, and you manage to route power into the safe (er... drilling the hole will decrease the fire safety), there is NO ventilation inside the safe and it will get very hot in there and the drive will die really quickly.
A variant of #1 is your option. But frankly what I would do as I'm paranoid is to have TWO drives I vaulted to. One in the safe, one connected so I could vault to it daily. I would swap the drives every week or two, and the one in the safe would be SAFE, not hooked up with power and some-such ;-)