"Hail, poetry, thou heaven-born maid, thou gildest e'en the pirate's trade."
The question is: what to do with "hail". In the next line Gilbert has his character say "Hail, flowing fount of sentiment, all hail, all hail, divine emoliant!"
Now, what is "all" -- an indefinite pronoun, or an adjective?
I guess it comes back to whether or not "hail" is an interjective greeting or the predicate adjective of an elliptical, imperative clause something like "let you be hail (i.e. healthy)"
"Heil Hitler" may pertain.
I guess I could ask more, but I am hoping for discussion.
Probably the etymology of "Hail!" could shed light on this.