Uhh no, I'm not a Dev and not the one who said that the engine does not support whirling weapons.
Did laying on the floor and kicking your feet while crying help as a child when told you couldn't have something? Someone lacked parenting abilities.
I'm glad you are not a dev. I am going to slowly explain it to you even though it has already been mentioned twice in this thread.
The entire slinging part can be recreated with a animation, you do not need a proper physics system for a sling. A simple animation with an arm spinning a sling would work perfectly fine. Much like drawing a bow, clicking would start the animation and when you release the button the finishing part of the animation will be played which consists of throwing a round shaped projectile. That is all you need, an animation, a sling model and a round projectile model. It is all perfectly possible within the Warband engine. I am pretty sure I have played a mod for Warband which already had slings, Brytenwalda probably.