I just did that, then found that "public" didn't really mean public; the post was still only visible to people with Google+ accounts. Oh well. I then copied over to my actual blog. But I would like to have a true public option even if it only meant read-only of such posts.
But just now I looked at the post and noticed a dropdown-arrow next to it which revealed an option to get a link. That link is indeed publicly visible. Also insanely long and ugly, but whatever, it works.