I know several people; all of them, in fact. Send a PM if you'd like a couple of e-mail addresses.
How is a residency program going to know what degree your mentor has? They really don't care. Do you think that a program that gets 800 applications goes through and says, "Oh, wow. She did research with someone who wasn't an MD. I guess the publications she got are totally meaningless." There's only one exception to the rule of "they don't care whom you do research with", and that's if your mentor is a huge superstar. If your research mentor has recently won a nobel prize, personally built a new spaceship that NASA will be using, or revolutionized the field of epigenetics with his/her research AND the person is recognizable by name, YAY! You've hit the jackpot and you MIGHT. You MIGHT. Let me say that again, MIGHT see some mileage out of the actual person you're doing research with. If not, then MD, MPH, MPA, PhD . . . Well, it just doesn't matter. In fact, it didn't matter to begin with because, like Dr. Neet said, it's all about the quality of the research.
With regard to the quality of the research, though, if you're going after publications hard, I would strongly suggest you ask about whether or not there's a chance for publication and iron out the details of that lab/clinical opportunity's policy with regard to publishing students. I've made the mistake of not doing that too many times.
Competitive medical students do well on their USMLE Step 1 and USMLE Step 2 exams and have excellent evaluations from their year 3 clinical rotations. A publication is awesome, but isn't necessary. If you want to go into orthopedics, dermatology, ophthalmology or another extremely competitive field, you'll want to do research IN THAT FIELD, preferably with publications. However, if you don't, just do some research, make a poster, enjoy your life, and don't worry too much. Having a publication is a gold star on your application, but is not something that is necessary.