I can't speak for anybody else, but I was having this problem of the zvol dropdown being empty -- and finally realized the problem was that I'd been creating datasets, not zvols, earlier. When I created a zvol as the docs say, it appeared in the list just fine.
Not saying anybody else did this -- but I was feeling all self-righteous and huffy about this obvious bug that everybody including me was having being closed as unreproducible, probable user error...and then I figured out what error I was in fact committing :-( . So I thought I should at least mention it.