Interesting question.
Requirements vary by state.
https://www.nass.org/sites/default/files/surveys/2020-10/summary-electoral-college-laws-Oct20.pdfIn Alaska, non-citizens would be ineligible since electors have to be qualified voters.
In Alabama, they're selected by a political party, so it doesn't seem they're required to be citizens, but it's politically unlikely that the party that wins its electoral votes will pick non-citizens for the electoral college.
California would seem a place where it could happen, and it seems Democratic electors are chosen by candidates for Congress, so it seems politically possible.