It amuses me that after having once made a bit of a name for myself as an outspoken critic of distance learning, I've gotten roped into teaching something called a "webinar," but it was for the best of reasons -- alumni! -- and it actually turned out great. Yesterday I had the privilege of spending an hour with 40+ Georgetown alumni on the web, discussing nonfiction book publishing and how to get a literary agent. Next week we'll meet again for a one-hour session on publishing fiction.
The best part of leading one of these seminars is the Q&A afterward, because the questions always surprise me. One author with amazing media credentials wanted to know if a literary agent would help her place articles in national magazines as a prelude to creating a nonfiction book proposal. For most people the answer would be no, because there isn't any money to be made on a percentage of the fee for an article. However in this questioner's case the answer is "It depends." A literary agent who happens to have a background as a subsidiary rights director in a publishing house (and there are a few of those) might have strong contacts at national magazines, so it might be worth asking.
Author credentials are the key, and this potential author had extraordinary ones. If you have them, great, but if you happen to be a mere mortal (aren't most of us?) then you will still want to land those major magazine or journal pieces, but you probably won't be able to enlist a literary agent's help. I'm not 100% certain that Ms. Media who asked the question will, either, but in her case I do think it is worth an ask.