In SLASH'EM, a severed hand is generally only seen as the base item for the Hand of Vecna. It has strong parallels with an eyeball. It is not randomly generated, but you can find a normal one in a bones level, or you can even wish for one.
The game will always ask for confirmation if one attempts to eat a severed hand ("Are you sure you want to eat that? [ynq] (n)"), as eating any artifact severed hand will do 150-199 damage. However, the only artifact severed hand is the Hand of Vecna. When eaten, a severed hand gives 40 nutrition points with the message: "This severed hand is delicious!". Oddly enough, it isn't considered cannibalism to eat one, it's just meat. A non-Hand of Vecna severed hand can't be sacrificed.