I think it could be doable -- are you talking about teaching for Kaplan, or somewhere else? I was going to do this before I started med school 2 years ago, but after going through the first training session for Kaplan, I really didn't agree with their teaching philosophy or the insane cost of their course. The curriculum is very regimented -- for example, you actually have to read lessons out of a book and you are not supposed to answer any questions that don't pertain directly to the test, which I really had a problem with (after all, these people are going to be doctors someday..if they have basic questions about biology, etc, I would like to be able to answer them). If your med school offers its own MCAT prep class, that might be a bit better..I'm not sure. I also felt like I wouldn't actually be helping the students that much -- it seems like people who can afford to take the class (I couldn't) would probably be sufficiently prepared that they'd do well anyway. No offense to anyone who teaches Kaplan -- this was just my experience, and I really wanted to do it mostly for the teaching experience rather than for the money.
As far as the time commitment, it would be a bit rough, but if you are efficient with your study time it could be doable. I can't recall if they had a specific requirement for how many classes per week -- I think it was something like 2 evenings a week or an evening and a weekend day. I got the feeling it wouldn't be very flexible as far as getting time off if you had an exam coming up, but again, I'm not sure about that since it was so long ago now.
In my opinion, free time is precious during med school, so I wouldn't recommend doing something like this just for the money. Live frugally and you won't go broke on loans. If you're interested in teaching, though, it might be a good experience. Just make sure you know what you're getting into.