This is how I imagine it would happen. A week before Election Day, a massive tsunami and earthquake forces residents and the government to temporarily abandon the state of Hawaii, after the infrastructure is decimated and residents temporarily evacuate to the mainland. As no residents can technically vote in Hawaii, the state's legislature votes in abstentia for the state's electoral votes. Just a guess.
I think you're right. AFAIK there's no federal law requiring electors to be chosen by the popular vote. It were states that gradually abandoned selecting them via legislature votes. South Carolina was among the last states to abandon the practice, if I remember correctly.
