No, they can't be combined
No, brass knuckles cannot be combined with either the Brawler or Martial Artist Edges. Even though the wielder counts as an Unarmed Defender, the attack itself is still 'armed'.
This is confirmed by Clint Black, the official Savage Worlds guru on the Savage Worlds forms here:
Brass knuckles are a weapon, and it doesn't matter that the user is considered an Unarmed Defender, the attack itself is not "unarmed," so the damage bonus from the Martial Artist Edge would not apply.
It is difficult to comment on balance issues for allowing it without knowing more about the setting you are running. It unquestionably makes the Brawler and Martial Artist Edges much more powerful because of the extra damage (average +3.33) it allows.