The book idea is an interesting and new way to do occulemency. The centaur was a bizarre and funny; it had the feel of as you were writing it she just appeared and you were just as shocked as Harry, which is a great way to write. I hope you find interesting things to do with her.
I get this mental picture of Harry, Luna, Susan and Laural all walking into the Ministry and when asked what their business is they say filing their marriage contracts. Of course the bigots and reporters leep to the wrong conclusions.