Handwraps of Mighty Blows can have runes, so your unarmed attacks benefit from these:
These handwraps have weapon runes etched into them to give your unarmed attacks the benefits of those runes, making your unarmed attacks work like magic weapons.
Can I use special materials on them as well, making my unarmed attacks Cold Iron or Silver?
The rules for special materials state the following:
Most materials are metals; they can be used to make metal weapons and armor. The GM is the final arbiter of what items can be made using a material
While "Most materials are metals" is not a useful statement, because obviously some are not, this strongly implies that special materials substitute similar normal materials. Special woods substitute normal wood, and special metals substitute normal metals. As Handwraps are "embroidered strips of cloth", metal can't be used to replace the cloth.
*Now, your GM might disagree and might allow it (The rules for special materials state that a GM is the final arbiter), but they should probably adhere to the pretty obvious intention behind the special materials. as laid out with the example of the special metals.
These handwraps have weapon runes etched into them to give your unarmed attacks the benefits of those runes, making your unarmed attacks work like magic weapons.
This magic item causes all of your unarmed attacks to gain the runes etched into the handwraps, but there's no mention of the same applying for precious materials. Without that your unarmed attacks wouldn't be affected by the handwraps' material.
Unless you're using the handwraps as an improvised weapon there's not much reason to craft them from something precious like cold iron or silver.
