Just to get a little heavy into some theory here - because I'm revisiting Morrison's Batman run in an attempt to reconcile Dave Uzumeri's insane but almost certainly with a lot of truth to them theories ...
At any rate. My annotations for Revenge of the Red Hood put heavy emphasis on my own personal theory that Oberon Sexton is The Joker's latest in a long line of disguises. Most of my reasons are spelled out in detailed lists within those pages, so I won't rehash them. Rather, I've decided to look at how Morrison has used The Joker - what role in the Batman world does he fill. If Batman is up against the "Ultimate Enemy", his personal Devil, whether Hurt winds up being Bruce Wayne after 10,000 years of Lazarus Crazy or whether Hurt is a literal Devil mind-screwing him ... how does Joker, Bruce's usual Rival, fit in? If the Arch-Nemesis isn't Joker ... and Joker is forced to take a spot on the side, what is that spot?
At that, we've seen in R.I.P. that if Batman is the proverbial "Heroic God" and he's fighting against an ultimate "Devil", Joker is more like a Trickster God - a Loki, to borrow from a familiar Mythology to comics fans. Thor's ultimate conflict is probably with Fenrir and Ragnarok (or Darkseid and Apokolips, if need be) but Loki ever plays the role of the wild card. And while I'm not as up on my Norse Myth as I should be to use that analogy, think "broad strokes" and you'll see how it compares to Joker, who the "Big Bad" wanted to use as a pawn, but who wasn't about to roll that way. And of course, even after Bruce Wayne is replaced by Dick Grayson, Trickster Gods are often depicted as masters of disguise.
Now ... specifically (and forgive me if this isn't the most neatly assembled essay, but I'm making it up on the fly) here are some instances and why they point me in a particular direction.
At first I took Harley Quinn's alias at face value. To be honest, Batman # 663 came out so long ago now that we weren't steeped in the insane Magnificent Bastardry of Morrison's grand scenario. And after all, Joker has employed the use of Trickster Gods before as aliases, much as any of Batman's rogues have used similar ruses, hundreds of times.
But upon further inspection (Source: Wikipedia):
"Wisakedjak (Wìsakedjàk in Algonquin, Wīhsakecāhkw in Cree and Wiisagejaak in Oji-cree) is the Crane Manitou found in northern Algonquian mythology, similar to the trickster god Nanabozho in Ojibwa aadizookaanan (sacred stories) and Inktonme in Assiniboine myth. He is generally portrayed as being responsible for a great flood which destroyed the world originally made by the Creator, as well as the one who created the current world with magic, either on his own or with powers given to him by the Creator for that specific purpose. His name is subject to many variant spellings, including Weesack-kachack, Wisagatcak, Wis-kay-tchach, Wissaketchak, Woesack-ootchacht, and undoubtedly others. Whiskey Jack". Jack is the widely "kind of" acknowledged first name of The Joker, first given to him in the 1989 Batman film (Jack Napier), and while throughout the comics history, often the "Play on jackanapes" Napier is dropped, Jack has lasted considerably throughout dozens of incarnations. I believe because writers inherently see the joyful fun of this man "Jack", the "Prince of Crime" (A Jack in a deck of cards is the Prince) being turned into "Joker", the "Clown Prince of Crime".
(Side note here: I've found boatloads of circumstances that point toward Joker being Irish, so the Whiskey part of the name has its humor as well.)
Morrison probably chuckled at how similar the word "Wisakedjak" sounds like "Wise-Ache" as well. And lastly, the Anglicized form became a nickname for the Gray Jay - a loud, obnoxious pest bird, which in another brilliant twist of linguistics, is a member of the Jay family. Jay, like "Mister J.", which is Harley Quinn's affectionate nickname for The Joker.
Now, to top it off ... Wisakedjak was used by one of Morrison's favorite writers, Neil Gaiman, in his book American Gods (probably where Morrison first read about it, to be honest). And if you think that's strange, consider the fact that Wisakedjak had an equivalent in another Native American tribe by the name of "Nanabozho", which is absurdly like the childish taunt "Nana" combined with the all-time most popular clown name of "Bozo".
Now, how that'll all link up to my theory that Oberon Sexton (Calling himself Oberon ... a reputable Trickster God, since he's King of Fairies) is Joker is circumstantial at this point. Apart from "El Penitente" (Probably Doctor Hurt) calling Oberon and trying to coerce his services in a way that could imply "he found where Joker is hiding out", there isn't much to go by. Oberon's book title (Masks of Evil). Oberon praising Batman and talking shit on TV about the "new" Red Hood, reminding everyone in Gotham that there was ANOTHER infamous Red Hood in Gotham long before this punk vigilante. His "background" of his wife killed, which while eerily similar to that of Michael Lane (Third Batman, current Azrael) is also eerily similar to a corrupted version of the already probably corrupted history Joker got in Alan Moore's "The Killing Joke". Not to mention how Oberon is head-to-toe covered in Black (Mourning the loss of Batman?) and even his eyes are covered by sunglasses. If he were The Joker, his eyes would be a big giveaway. His new cut smile would need a full black mask to conceal. And the top hat doesn't hurt. Joker has been known to wear top hats.
I've been trying to draw some connection between Oberon's mention of the "Riddle of the Corn Dollies" and the Cree Indian Legends, but frankly ... "corn" in Early Europe meant grain - the big, yellow cobs we have here in the states were called "maize" by the Indians. So other than my own wish fulfillment thinking of Joker knowing something of his own Irish roots (Corn Dollies while not unique to the British Isles, certainly seem to be dominated by British, Scottish, Welsh and Irish lore) I had to turn to the legends.
The pre-Christian settlers of the British Isles had a harvest folklore that said there was a "corn spirit" living in the fields and whenever all the corn was scythed and harvested, he'd be homeless. Joker right now, without a "real Batman" to duel with, is sort of homeless.
I admit though, this line of reasoning is grasping at straws ... (bad pun). Anyway, there's no real through-line of connection there, but I wanted to at at least try to explain away the whole Corn Dollie mention, because if it IS The Joker, I'm hard-pressed to see why he'd bring up something that seems so much more up The Scarecrow's alley.
Last for now ... there's the matter of Joker evoking old pagan gods. Trickster gods like Wisakedjak, or Oberon or any other, are all pagan gods. Before Christianity came along, and before most of the themes with which Morrison has been playing with (good and evil, god and the devil, Christ-like Batman) existed. They were used by primitive (cowardly, superstitious lots) people to explain forces of nature. And Joker is a force of nature.
I can't help but wonder if Morrison, in his "Ultimate Battle Between Good and Evil" in the pages of Batman sees Joker as something of a pagan influence - if Batman represents "Good" and Hurt represents "Evil", perhaps Joker is here to represent something much older and much more adaptive. Perhaps the reason why The Joker doesn't take The Devil seriously is because what he represents is older and doesn't fear these newfangled Judeo-Christian concepts.
Before there was ever a Satan, there was hundreds of Tricksters, toying with Heroes for much simpler, pettier, REAL HUMAN reasons better than any kind of Good vs. Evil, Discipline vs. Temptation religious philosophical battles. And those Tricksters will outlive Good vs. Evil. In fact, Judeo-Christian Religion is dying in the modern world of Reason and Science. And that's why Joker considers himself ahead of the curve - the perfect evolution of crazy. In a fast-paced world, he's even faster. And he's cast aside outdated concepts rooted in Judeo-Christianity like "Right and Wrong" in favor of being "Unreasonable in a world of reason", "Hyper-Logical", and "Whatever the fuck he can get away with.