Yeah I think its possible, but just keep in mind it will be very difficult to have a substantial project.
I started a lab rotation for the MD/PhD program this summer, and decided to stay in the lab during the school year and put in about 10 hours/week finishing up a couple of experiments.
My stuff is imaging though, so thats kind of a special case. For some wet-bench type experiments, they can be much more time demanding, so it depends a lot on what kind of research you are talking about.
Most people at my school dont do research during the school year, but its possible if you have a good handle on what kind of project you are going to work on and can devote maybe 10 hours a week to it.
Depending on the school you attend and the structure of the curriculum, research may or may not be easier to work on throughout the year. A few hours here and there might be feasible, but I'd definately rule true bench research out. Most people I know who did "research" during the year used the time to finish up old experiments and tie up any loose ends. Still, depending on your school, anything's possible...