See, that's the best part. You can make shit up to fit. *grins* You know what's available in canon, so you can mutate/shift that ability for Zack.
And I'd totally read that AU of an AU. But... wouldn't that mean Zack would be killed dead by Itachi? Or would Zack be able to survive Itachi? *ponders* Or would Itachi even do it, with Zack around? .... damn but Zach has bad luck with the genius-types.
...Oh but wouldn't it be? *___* And dude, he'd totally have a fanclub, a la Sasuke. So would Zack, for that matter. And that Sephiroth would is a total given.