Maybe this is grasping for straws, but one possibility is Billy Batson/Captain Marvel. Note Maggin's specific, unusual wording: "the secret identity of a major character" instead of "a major character." Is it possible the idea was that Captain Marvel is straight, but Billy isn't?
You're certainly tickling my funnybone there, Julian, because I have always liked the idea that Captain Marvel and Billy should be different people - in fact, I think it's essential to the character(s). However, I think you're letting your imagination run away with you, as indeed was I. A gay Green Lantern isn't so far-fetched, though, after his experiences under Denny O'Neill; I shouldn't be surprised if that's what they had in mind.
Dilber MC Theme by HarzeM