I went to Northwestern's School of Continuing Studies Post-Bac program, which is Chicago's equivalent of the Harvard Extension School.
Like Harvard's extension program, Northwestern's SCS program caters mostly to career changers that attend evening classes to complete pre-med and pre-dental basic science classes. Many of these students are still working day jobs, but some have quit their jobs to finish this program full-time.
Like the Harvard program, some Northwestern day students and faculty have a
"stigma" towards the SCS program. However, the SCS program is well received from Medical and Dental schools as evidenced by the fact that a high number of SCS graduates matriculate into prestigious medical dental schools across the country.
I am certain, that the Harvard Extension program places a high number of students into prestigious medical and dental schools.
Like dsony2284 stated, both the Harvard and Northwestern name attracts the brightest and most dedicated post-bacc/career changing people in the area, creating a highly competitive student body. As a result, it does look good if you can do well at a program like Harvard Extension.
Knowing what I know now, if I live in the Boston and need to take d-school pre-reqs as career changer, I would attend Harvard Extension. I can care less about Harvard's cost of tuition.
One last thing, like the OP, I was a career changer from engineering. Northwestern helped me get accepted to four d-schools.