It appears the permanent broom mount will be available through the Trading Post.
While we thought the Trading Post source on Eve’s Ghastly Rider was just a placeholder, it appears Blizzard plans to make it available via the Trading Post as opposed to having it drop from the updated Hallow’s End event.
A new flavor text was added for the mount in the latest 10.1.7 PTR build. It refers to the Fenlow family (some Hallow’s End quest vendors).
“The preferred transportation of the Fenlow family (especially during Hallow’s End).”
The mount is instant cast during Hallow’s End, so we expect it to be on the Trading Post in October.