I've got some house rules, but I've yet to see some official stats.
Quoting from my house eratta:
"Atlata: The atlata, or spear thrower, is a weapon designed to increase the distance with which a person may throw a dart or javelin. Essentially it is a stick with a cup designed to fit the base of a dart or javelin. When properly wielded, it acts as a lever and darts and javelins hurled with it are treated as projectile weapons. The maximum distance for darts or javelins thrown in this way is 10 range increments, and the base damage from such weapons is increased to 1d6 for darts and 1d8 for javelins. Atlata’s are largely obseleted by the invention of the bow, and as such are principally used by primitive tribes who either lack knowledge of the bow or the skill to create one. Atlata's may not be wielded without proficiency. It requires an Exotic Weapon proficiency to use an atlata with any degree of effectiveness."
I've never actually used an atlata, so I've yet to decide whether loading the atlata is sufficiently clumsy to be considered a move equivalent action like an xbow, but since it has not come up I'm not worried about it. Essentially, you get a bow with a shorter range and lower critical multiplier, so this is a DM only toy for providing atmosphere.