I've done it (add sticks) with a pumpkin beer years ago. It did give flavor, though probably not as much as if you use ground cinnamon. It's also a gradual release of the flavor, so you won't have an immediate impact.
The best way to control the amount of flavor would be for you to make a tincture (cinnamon and vodka) and add the cinnamon vodka to the keg until you get the flavor you want. I don't care for tinctures, personally, but they work. And it will keep the grounds out of your keg.