I think a pre-processing directive occurs before the new part record is actually created, and a post processing directive is too late to change anything. So, in my DuplicatePart post processing directive, I have to first fill a table with the new part record, then set the fields the way I want, then Invoke Part.Update BO.
I am also deleting rows from the part revision table and formulating an email to send to production/purchasing which make this look more complicated than you need just to change a few fields. The 5 widgets highlighted may be all you need.