Regarding his trait

HARLEY'S MY GIRL: If a friendly Harley Quinn took damage from an attack since your last turn, you may give The Joker a free action. If you do, place The Joker adjacent to a character that caused Harley Quinn to take damage and target that character with a close combat attack. Harley Quinn is dealt one unavoidable damage. If Harley was KO'ed during your opponent's last turn, The Joker's attack value becomes 12 until the end of your turn.

I have seen previously that you cannot activate this ability using mastermind, since mastermind damage is not an attack. However, the activation clause is separate from the clause that allows you to place The Joker. So long as at least one of your opponent's characters attacked Harley directly, would you be able to place The Joker next to a character that he masterminded damage from?

They're not separate. The sentence regarding the placement starts with "If you do..." wich is a condition of using that free action mentioned in the previous sentence.

Let me see if I'm understanding you correctly. There are opposing characters A and B. A attacks Harley and hits her for 1 damage. B attacks Joker, who then Masterminds 1 damage to Harley (A and B are both pretty weak characters).

You are asking if Joker can be placed next to character B, right?

I don't think that would work, because even though the damage came from characters B's attack it is Joker who caused her to be damaged with the use if his Mastermind.

That answer makes sense.
I saw a recent clarification on Mastermind in the 2013 player's guide that states

• Damage dealt by Mastermind is consider to have come from the original attacker.

I'm trying to find places where this is actually relevant, seems this isn't one of them.

You need to look at Joker's power like it was Mystics. You need an actual direct attack to be able to pull this off I believe.

(Can I get an Orange to say that I'm wrong?)

No, but you can apparently get a heaping helping of spam, so enjoy that.

It's convoluted but yes, I think it would work the way the OP is thinking. If char A deals harley one, Char B hits joker, he MM's it to harley, since the dmg is considered from the attacker, ie B, B is "a character that damaged harley quinn".