A UCL "unlawful" prong hypothetical

A reader writes in with the following hypothetical:

Are you aware of any California cases addressing basing a UCL claim on federal law when there is a California law that expressly allows conduct? Given that federal law trumps state law I could see an argument that the California law is just invalid. At the same time, I would think that if a specific California law permits a practice, the UCL (as a more general California law) cannot make it invalid.

For what it's worth, I think this is an issue left open by the Olszewski case where conduct violates a federal statute, is expressly allowed by a state statute, but the state statute arguably is preempted by the federal statute. The question is whether the Cel-Tech safe harbor applies, which requires the answer of two questions (1) does the Cel-Tech safe harbor ever apply to "unlawful" prong cases; and (2) if so, does it apply when the state law that creates the safe harbor is preempted by federal law?

What do you think?