Dear Mag Writer…
I presume the magazines are editorially discriminating and of professional quality. After all, you’re not just touting your writing chops, you’re plugging your potential as a business colleague, too. I support mentioning such mag writing in the credentials portion of your letter. If you’re a debut novelist, you can stand to cite evidence of your chops and professionalism. Ring this bell extra loudly if the magazines’ subject matter jives with your novel’s subject because this establishes your expertise with that topic. A caveat: If the mags are political or in some other way topically sensitive, only mention them if the topics are relevant to the book you’re pitching. Or be general about them: “This is my debut novel, although I’m published in magazines.” You can share the deets of nonproject-related interests in later, deeper talks as necessary and appropriate. For now, pitch every hook you’ve got.