Why isn't it viable? People with lower GPAs do it to make up for their lower college gpas, so I don't see why it wouldn't work for mcat scores. It just seems like another way to distinguish yourself from other applicants and show stuff your mcat score may not have shown.
If someone else already answered this, I apologize, I didn't see it.
Disclaimer: I am not involved with any admissions process, so this is speculative.
Admissions committees partially choose applicants based on how likely it seems that the applicant will be able to perform as a student: two indicators we all have for this are our GPA and our MCAT. The difficulty of attaining a good Gpa can vary a great deal by institution and major: in my experience, it is often more a matter of motivation/study skill than ability (anecdotal, subjective, and biased experience). It is also measured over four years, which allows for trends: having an upward trend is obviously helpful. Because gpa is influenced by these other factors, and measured over such a long period of time, a masters can help in several ways: the fresh start in graduate school gives you a fresh start to improve your GPA, the fact that it comes after college allows for an upward trend, and the more difficult nature of the classes better indicates your ability to survive rigorous classes like those offered in medical school.
The MCAT is a totally separate beast. It is often given so much weight because it is a relatively objective, constant measurement of your basic knowledge/reasoning in comparison to your peers (although far from perfect). No matter how well you do in your masters program, if you do terribly on the MCAT, it indicates a poorer understanding of the basic material than your high scoring competitors. Your masters may still help, as it indicates you can tolerate graduate level coursework, but it cannot replace a powerful foundation in biology, chemistry, and physics. It may even hurt you if your score is poor in an area corresponding to your masters (if you are a graduate physics student, I would expect you to destroy the physics portion).
So to my understanding, getting a masters degree may make up for a low GPA, depending on your performance in the program, but it is unlikely to help as much in repairing a poor MCAT.
Also, masters programs are really expensive, and far from easy. Don't get one unless you have a plan to use it and are passionate about the subject matter. For MD admissions, it seems to me, you would be better off getting experience showing your continued interest in health care. Especially if you can make money and study for an MCAT retake while doing so.
Tl dr: masters helps with gpa due to trend and difficulty, does not help with MCAT as it still indicates a poor base understanding of knowledge needed for med. school. There are better uses of your time than getting a masters if you only plan to use it for your application.
