Relationship designators are not core (not required) in RDA so you do not have to use them for every authorized access point in a given bibliographic record. In other words, it is not an all-or-nothing proposition. You could have two AAPs with relationship designators, and three without any.
I don't think "writer of added commentary" is appropriate in this case, as it is defined in RDA as "A person, family, or corporate body contributing to an expression of a work by providing an interpretation or critical explanation of the original work." I can see the argument for the author of a preface or introduction fulfilling this role in some cases, but the connotation of RDA's definition doesn't "feel" like it fits to me.
While there is no consensus on what relationship designator we use for the author of a preface, introduction, etc., the MARC relator code list has "Author of introduction, etc."
which seems reasonable.
I've heard one objection to using this as the relationship designator because the "etc." implies that the person wrote more than just the introduction. I still prefer it to "writer of added commentary" or to making up uncontrolled terms for relationship designators (although that is what RDA instructs us to do).
Jacob Burns Law Library
George Washington University