Eh, making it too weak isn’t a good Idea. I would suggest Iron Durability and Quality. That way it could be an Iron Tool Replacement in the Nether. 
Btw when you mentioned replacing the EntityItem Class for Fire protection, I thought there must be a better way, and indeed I found a better way:
if (!aEntity.isDead && tFireProof && aEntity.isBurning()) {
UT.Reflection.setField(EntityItem.class, aEntity, "field_70291_e", 250);
}
Its GT code but should be readable. Basically I use reflection to set the Items Health from 5 to 250 every tick, whenever it is on Fire. I just tested it with Lava a few minutes ago, it works. XD
Thanks for giving me the Idea to look at it, so now I have Fireproof Materials in GT6 (everything made of Efrine would count for that too ofcourse, I have multiple such Materials)