Will Baude asks, "Can slightly less addled readers than I name Supreme Court cases where the court held that a state constitution was held to violate a federal statute? I'm positive such cases exist-- under the Voting Rights Act if nothing else-- but nonetheless have been oddly unable to enumerate any..."
I couldn't think of any myself. Dalton v. Little Rock Family Planning Services is the closest case that comes to mind. In Dalton, the Arkansas constitution was amended to prohibit use of state funds to pay for any abortion other than to save the mother's life, and this constitutional provision was enjoined by a federal district court due to conflict with Title XIX of the Social Security Act, which in conjunction with the Hyde Amendment permitted funding for abortions that resulted from rape or incest as well as those to save the mother's life. But the Supreme Court decision was to limit the scope of the district court's ruling to enjoin the state amendment only with regard to Medicaid funds.
State constitutionally-mandated distribution of representation seems to get challenged more under the 14th Amendment than the Voting Rights Act (Brown v. Thomson), though I'm surprised that Arizona's attempt to amend its constitution to make English the official language didn't run afoul of the VRA's language requirements. I suppose the amendment was tailored to fit with the VRA. States generally have the sense to amend their constitutions in accordance with federal law, rather than try to challenge it (Three Affiliated Tribes v. Wold 1984), and federal interpretation of rights sometimes is more popular than the state's. See Fla. v. Casal, where the Forida electorate amended their constitution to say that it grants no more rights than the federal 4th Amendment, and a similar move in Crawford v. Bd. of Educ., where the state constitution was amended so that busing demanded by that state constitution couldn't exceed that demanded by the federal courts.
I'm surprised that the Republican Congress did not pass a federal statute in response to Locke v. Davey that specifically prohibited states from discriminating against religious education in granting scholarships, which statute then would have overridden the Washington State constitution. It seems like it would have been good red meat for the conservative base that is convinced religious people are oppressed. And if we had a federal anti-discrimination law that applied to sexual orientation, that would have killed Colorado's amendment in Romer. In Gregory v. Ashcroft, the Missouri state constitution's requirement that judges retire at 70 was found not to violate the ADEA because the ADEA was assumed not to cover appointed state judges.
I'm also surprised that we haven't seen a lot of state constitutional amendments created by intitiative later found to be violating federal law, given that the federal courts can't give an advisory opinion on such initiatives before they become law.