Harsher penalties for a specific circumstance.

That's not really analogous to revenge porn laws (where the “revenege" part is both non-literal—the actual condition is lack of consent—and refers to a special circumstance that makes what is normally legal, illegal, not an enhanced penalty for existing offense.)

But if your proposed concept of “revenge libel” laws are just, as you say an added penalty for a subset of existing libel offenses, then while they might add more severe sanctions, they don't change the scope of what is prohibited, so they wouldn't change the calculus on whether anything is illegal.