I've created a Cold Iron and a Silver variant of the Brass Knuckles in case I have to fight Fey or Werecreatures. The Silver ones have been used and since they are blunt weapons they do not suffer the -1 penalty to damage.
Enchantments on the weapons themselves allowing them to do Monk damage could be unbalancing and at 4th level you can hit creatures needing magical weapons anyway. For organized play I doubt I'll see any enchanted Brass Knuckles.
Monks do not care whether or not they come in pairs - I assumed that you buy one at a time. A flurry of blows can all come from the same fist so the point is moot.