As you have mentioned (and we discussed prior to the publication of your essay), Campbell doesn’t really express himself on the subject – though I do recall a couple passing mentions re life in outer space (apart from the Fire Women from Outer Space poster). I have looked for those since our email exchange, and finally found one:
The idea that we will have a divine visitation by some friendly forms, benign forces from other planets who will come to our aid and save us, is a clear reflection of an outmoded understanding of the universe. Jung wrote that the modern myth of unidentified flying objects tells us something of humankind’s visionary expectations. People are looking for visits from the outside world because they think our deliverance will come from there. But the space age reminds us that voyages into outer space turn us back to inner space. The Kingdom of God is within us, but we have this idea that the gods act from ‘out there.'” (A Joseph Campbell Companion: Reflections on the Art of Living 168-169)
The other reference, which was much warmer toward the likelihood of life elsewhere in the universe, I’m still searching for.