User blog comment:Thailog/Ep. 205 "Beneath" discussion/@comment-4821105-20120816103448/@comment-1038387-20120816112404

From #21:


 * Aliens have arrived on Earth-16!
 * Superboy and five Justice Leaguers have been abducted!
 * Metropolis is in danger!
 * Enter: BRAINIAC!

It doesn't say Brainiac does the kidnapping. In fact, the solicit for #22 gives a better clue: Kylstar.

Kylstar, in his one comic appearance (guess who (co-)wrote it), kidnapped heroes to let them fight against the dictator of his planet. He's a crystal-based lifeform, and he stored the heroes on crosses like the ones we see on the cover of #21.