Aquaman makes a surprise appearance in Frank Miller's Dark Knight III: The Master Race series--specifically, the Batgirl #1 tie-in comic. As Batgirl is being chased by a bunch of armed and violent goons, she finds herself at the end of a pier, and waiting for her is the Sea King:
...to be continued!
To my memory, this is the only time Aquaman has ever been drawn by Frank Miller. He didn't appear in The Dark Knight Returns (boo) or in The Dark Knight Strikes Again (yay), nor in any other Miller-penned project during the man's long career. So when I saw Aquaman pop up here, I was quite surprised. I'm guessing this means he'll be back for Batgirl #2!