I have an odd writing credential. I write historical romance now, but in the early 80's I had several (mostly humorous) articles published in a book by one of the major houses. However, my name is not on the cover. This was a compilation of articles supposedly written by women from all over the country on topics of relevance to women. In reality, 3 friends of mine (the ones with their names on the cover) asked me to help them write the stinkin' thing, since they had almost zero response from women all over the country and a looming deadline.I wrote articles like "How to flirt" and "How to do a strip tease", most of them pseudonymously. Is this something worth mentioning in an agent query? Like "I had several articles published in (name of book, name of publisher)"? Or does this just seem like grasping at credentials?
Miss Snark attributes much of her success to reading that article on how to do a strip tease.
Much like the vaunted but anonymous "fixers" brought in to remedy ailing plays on Broadway, this work may have saved the writers' bacon but it's not attributable to you. Play doctors get a reputation for being the "go to girl" when things fall apart in the second act, but no credit on the program at all. If you were to continue to do this kind of work, you'd find your name being circulated by people looking for such "fixers"; we keep a rolodex of names of people who can do interesting and unusual things just in case.
As it stands, chalk this one up to doing a good deed. The universe owes you one!