The theory stems from a shooting schedule for Ring Around The Riddler in which Siren's brief walk-on is alleged to be portrayed by Ertha (sic) Kitt. Now, keep in mind, that it's doubtful that anybody in a position of authority at Greenway would've made such an egregious mistake on Eartha's name. The multi-talented Ms. Kitt was super-famous at the time.
Thanks goes to fellow board member epaddon, who unearthed a New York Daily News story from Friday, June 9, 1967 (attached). It states that Kitt had signed on to star in a hoped-for Broadway miusical titled "Peg." Unfortunately, it never made it to Broadway. As has been documented on this board before, Kitt suffered a fall on stage during a Northeast pre-Broadway tour of the production and suffered a back injury.
But the point is this. She was committed to the production in early June. In fact, the story also mentions that from Aug. 1-13, the show would be at Westbury Music Fair on Long Island, not far from New York City. Thus, she would have been on the opposite coast when Ring and The Wail Of The Siren were going before the cameras. And the tour already was ongoing when the Olga/Egghead arcs were being filmed. The point is, Eartha wouldn't have had time until MUCH later to do Batman. (Also, the first two arcs filmed featured Burgess Meredith. And as we all know, they ALWAYS worked around his schedule. That is well-documented.)
As Bob Furmanek posted in a thread several years ago, both Eartha and Julie were announced around this same time as being signed to appear as third-season villains:
viewtopic.php?f=2&t=6277&p=60893&hilit= ... itt#p60893
But certainly the folks at Greenway had been advised by Kitt of her plans, and her availability. Maybe they were keeping their options open in case Newmar came back. I don't know. I do know the first draft of Ring was dated June 21, 1967, and by then, it would have been plainly obvious Kitt wasn't in the mix for any villainess role until at least the fall in terms of shooting. There's no way to confirm Stanley Ralph Ross' quote that he wrote Siren specifically for Joan Collins, but his treatment described Siren's singing voice as painful to the ears, not melodic. Not a description of Eartha's vocal talents, that's for sure.
My hobbies include gazing at the Siren and doing her bidding, evil or otherwise.
'She had a devastating, hypnotic effect on all the men.'--A schoolmate describing Joan Collins at age 17
-- The Joker, in a line cut from "The Joker's Epitaph"