I actually know some med students who got their masters in psychology during elective time (or maybe they took an extra year, I forget). I was a humanities major as well, I think it helped a lot regarding acceptance.
Humanities majors actually have a higher rate of acceptance to med schools. This is a fact. Significantly so actually (there could be many reasons for this, another topic). Also easier to boost your overall GPA taking less hardcore science. Think about it this way. Traditionally, people went to college to learn how to "think". They learned a bunch of topics that people would now regard as less than practical like latin and philosophy. But that is what college is for. It is the only great opportunity you have in today's society to learn what you want, a true "American Scholar" a la William James. Be happy about your psychology under degree. You'll have more than enough biological sciences in the future.
Your MCAT scores may not be as great, but who knows. Take some practice tests after you finish your prereq's.
The road less traveled, and all that stuff...We always need more inspired people in the child world.
Oh yeah, the whole point of GPA and MCAT is to score the interview. At most med schools, you have a 50% chance or so after you get the interview (most of weed out is prior to this). So do enough to get the interview and dazzle them with your well rounded education. When they discuss specialty of course, say you are open minded to not offend other specialties. Good luck!