Capable of repetition yet evading review exception to mootness is an equitable consideration; as the faithless elector could have filed suit during the vote-casting process, the exception is not available to him now.
http://media.ca8.uscourts.gov/opndir/18/09/164551P.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.