"18 U.S. Code § 1466A - Obscene visual representations of the sexual abuse of children
Any person who, in a circumstance described in subsection (d), knowingly produces, distributes, receives, or possesses with intent to distribute, a visual depiction of any kind, including a drawing, cartoon, sculpture, or painting, that (1) (A) depicts a minor engaging in sexually explicit conduct; and (B) is obscene (...) or attempts or conspires to do so, shall be subject to the penalties provided in section 2252A(b)(2), including the penalties provided for cases involving a prior conviction. It is not a required element of any offense under this section that the minor depicted actually exist."
Those two words are hiding a lot. For example, it's probably not obscene in Oregon, since part of the Miller test defers to state law and we have a stronger state equivalent of the 1st amendment in our state constitution that would allow it.
And if you wrapped it in a story(like a manga or comic), it would be easier to argue that it has literary or artistic merit. Though, a "states' rights" argument would probably be more likely to succeed.
My comment wasn't meant to speak to the test of the federal register, under which a huge part of the everyday lives of Americans are federally prohibited.
I was more speaking to the question, "is it a crime?" IE, is it a crime in any sort of common-law sense and the proper purview of a government in a functionally free society.
In that sense, I do not believe that the wholesale fabrication of any form of media is a crime.
Any person who, in a circumstance described in subsection (d), knowingly produces, distributes, receives, or possesses with intent to distribute, a visual depiction of any kind, including a drawing, cartoon, sculpture, or painting, that (1) (A) depicts a minor engaging in sexually explicit conduct; and (B) is obscene (...) or attempts or conspires to do so, shall be subject to the penalties provided in section 2252A(b)(2), including the penalties provided for cases involving a prior conviction. It is not a required element of any offense under this section that the minor depicted actually exist."