It's really little like other suggested Mars missions...it's a bare-minimum mission with delta-v requirements considerably lower than other alternatives due to the free return trajectory. The only propulsion needed after departure is for correcting errors in their trajectory. It's all minor variations on equipment that we expect to exist by that time.
However, that free return trajectory means they've got one small launch window in the relatively near future. Even if commercial crew development goes as planned, that leaves them with a small period of time to develop and test the vehicle. If they can't launch on time, their system won't be able to make the trip.
Think of the consequences of putting a bowel movement a day by two people overboard for 500 days (that's a thousand movements at least) on a mission without any major orbital changes and you can work out why jettisoning it is not going to be a good idea.
1000 very small comets drifting just outside the windows.
Really, it should be doable. It wouldn't take much of an impulse to get refuse well clear of the spacecraft. But it'd require an airlock just for chucking garbage out of the spacecraft, designed to reliably eject it at some minimum velocity and be cycled perhaps hundreds of times over the mission, and the main result would be less shielding. Jettisoning refuse might be done for a spacecraft that has to brake to enter orbit, but this is just flying by.
