In my experience, HES post-bac will not help get into HMS. However, despite what one poster said, it would not be bc you "had" to do a post-bac. Plenty of post-bac students do get into top medical schools - especially those post-bacs who are career changers. People who do post-bacs to change careers are different from those who need to improve their GPA.
At some places, doing a post-bacc in the geographic area of a medical school might help by showing a motivation/dedication to live in a certain city, but Harvard will not care. They know people will want to move to Boston to go to HMS.