However, I am a naturally androgynous person, and I feel personally put-out that Gibson went out of his way -- using a female actor with a dubbed male voice -- to use the image of an androgynous person as the embodiment of evil, deceit, and unnatural being. I suspect, actually, that he was influenced by Marilyn Manson.

I think you are oversensitive on this issue. Why can't you accept the possibility that Satan was depicted as androgenous because that's the common Christian perception of angels? Lucifer is a fallen angel, thus he would be depicted as androgenous. Why the male voice? Because Satan is often thought of as a "he" as are most angels.

I don't believe that the imagery in any way was meant to be homophobic.

Maybe there's something to your "subiliminal" theory, but Mel Gibson is nor more guilty of using subliminal messages in the way your describe than Judas Priest or Ozzie Ozborne were of putting subliminal messages in their music.