I'm not denying any possibility. I'm too confused about Batman Who Laughs' six secret sleepers right now to argue anything.
I thought that the six sleepers are characters from the main continuity who don't know at the moment that they are controlled by Batman Who Laughs.
But looking at the
preview of Batman/Superman #1 and the solicitation of #2 with evil Shazam who's from the dark multiverse, I'm not sure anymore that any character from the lmain continuity will be used.
(And looking at the preview, Shazam doesn't fulfill the definition of a sleeper to begin with.)