You could also leave the weaponry alone and make a parallel set of Armor promotions, which are opened up in a seperate promotion line (doesn't have to be magic, I'd say Priestly if anything). Still using the WeaponTier of the unit to decide if they can use it, and the base resources, but now granting defensive strength and/or immunities. Thus if you have melee and priest lines maxed out, you have Mithril Weapons + Mithril Armor on all of your appropriate units, granting a net +4/+6 with 50% resistance to all damage types and magic (or somesuch)
I guess an advantage here is you could have iron weapons and copper armor.
Heck we could build an entire equipment system, with priest giving armor, mage giving rings?(magic resistance or something), recon giving something else, etc
