I think these weapons (brass knuckles, cestus, gauntlet, and similar) need to receive a special weapon property called unarmed which allow the user to treat the weapon as an unarmed strike for purposes of weapon proficiency, ki strikes, monk unarmed damage.
I suggest this on two factors: First that the attack motion between an unarmed punch and a brass knuckled punch is for all purposes identical, and that all other things being equal the extra mass of the knuckle should not be less effective than that of flesh and blood; Second that if the minimal material these "weapons" consist of blocks the monks ki then so too would the leather shoes the monk is wearing, or his pants, or his robes or his headband.