You have to remember that the most safes aren't rated as "fireproof" but as "fire resistant." Big difference. And as Jorge pointed out, while the safe will protect the item from direct contact with the flames - that interior gets, mighty, mighty hot. I'm not sure if you can get an insulated safe that will keep the interior temperature at a point that wouldn't effect the film emulsion.
I'm all for empirical testing for these type of questions. Why not take some scrap negs, put them in the oven at 400 F & see what happens? Then you'll know what to expect and whether the safe will really do you any good.
If you do test this, I'd like to know the results.