I don't think the player should have any direct control over cannibalism. Dwarves don't do that in normal conditions. Of course, having cannibalism in the game would immensely further the cause of unnecessarily gruesome realism, bot you shouldn't be able to simply order someone eaten, unless the race you are playing has no ethical problems with cannibalism.
At the moment, starving dwarves catch and eat vermin. It would be nice if this was extended to unprepared fish, unbutchered animal carcasses, rotten food, old boots and, possibly depending on personality traits, people. Cannibalism could also be tied to going berserk. Berserkers have already stopped caring about ethics or society or other silly things like that, and might as well eat the things they murder if they happen to be hungry.
In short, cannibalism should be like murder; something the dwarves just do occasionally with no direct player input.
You couldn't designate dwarves for buthcering, but you could still get some dwarves eaten, much like you can have dwarves murdered if you try. Just drop a bunch in a hole wihtout food and hope one of them has the required personality to kill and eat the others.
Oh, and related to this, if your race had no ethical qualms with cannibalism (or murder or whatever), you should be able to designate people for eating (or killing or whatever). It might still cause unhappiness, if the deceased was friends with anyone, but you should be able to do it.