Note that mobs can't spawn on bedrock or thin air. If you make sure to cover whatever structure you build on the roof of the Nether with glass or any other spawn-proof block, or drop a Magnum Torch nearby (or maybe light the place up; I'm not sure what light levels ghasts require to spawn), neither ghasts nor pigmen will be able to spawn there, so you won't have any trouble.
Even if you do that, you might have trouble if there's a pocket of lava right below the bedrock ceiling, but I don't think there's enough of those to set very many things on fire.