The new, evidently terrible Batman vs Superman movie turns on Lex Luthor's evil plan to lobby the US government to grant a variance in its import controls on kryptonite (making the movie part of the pantheon whose creators bravely decided to make the major plot points revolve around regulation, see, e.g., the Star Wars prequels).
The Law and the Multiverse blog (which uses comics, science fiction novels, and superhero movies as case-studies for understanding the legal system) (no, really, and they have an amazing book that's practically a first-year law-course made out of comic book examples!) invited Lawrence M. Friedman from the Customs Law Blog to weigh in on the ins and outs of import regulation, and providing pro bono advice to Lexcorp on getting its exception.
Lexcorp's best option might be the research and development exception of 40 CFR § 720.36. This still requires that the amount imported be small and it also requires notice to employees of the risks associated with the chemical. Given that kryptonite has no apparent negative consequences for humans, that appears possible. But, at the point in time depicted in the movie, it is not clear that the full consequences of human exposure to kryptonite are known. Lexcorp should be careful about that.
On a related front, if Lexcorp is able to import kryptonite, it may have to contend with the Hazardous Substance Act, which regulates, among other things, the labeling and packaging of hazardous materials. Again, based on what we see in this movie and 75 years of Superman lore, it appears that kryptonite may not be hazardous to humans. So, this may not be an issue.
Batman v. Superman and Import Licenses
[Lawrence M. Friedman/Law and the Multiverse]