I'd say the main reason that it won't work is the same reason that so many of us convert our chest freezers to keezers by making collars for them... the coolant lines. Its difficult, if not impossible to figure out exactly where the coolant lines run because they are integrated inside the walls of the freezer and in your proposed situation you would have to know where they were so you could split your freezer into a "refrigerator section" and a "freezer section".
Also, I'm betting that even if you could figure out where the coolant lines were running, chances are you would find that they would, at the very least, run the length of the back of the freezer thereby making the task of separating the freezer into two different temperature hemispheres impossible.