Can I change the fire rating of 1/2 inch drywall?
Intumescent coatings have evolved (I formulate them). It is just a paint now. 3 coats would add 20 minutes or so to the fire rating of the drywall.

Here, narrow clearance homes are required to use intumescent coated OSB on the outside walls. 

These paints can withstand up to a year of exposure before being covered up so bad weather is no longer an issue as it used to be just a few years ago.

The old stuff was PVA resin and water soluble (almost edible) ingredients. Now we have encapsulated stuff with tough, exterior grade thermoplastics as the binder. Things have changed :)