You get a +Rare Drop and +EXP bonus in the Halloween Urgent Quest for wearing any of the outfit pieces listed in this article: https://pso2.com/news/announcements/scarewear1014

However no cast parts are included or mentioned. Do Casts just have no ability to get this bonus effect?

CASTs can wear most of the unisex outfits, all of the outerwears and basewears.

Oh wow I did not realize that, thanks!