My use case requires me to do this, but I believe its besides the point. An atlas would just shift the tedium over to creating 400 individual images (which doesnt even make sense when working with such highly symmetrical assets), naming them all appropriately, and going from there. In fact, it may be more tedious (correct me if im wrong, i dont have any experience with the atlases.)
Im of the firm opinion that anything in the engine should be easily manipulated programatically, and that the defold interface should only be a convenience. It shouldnt take center stage to the point of forcing one to build games around the interface, at least not for such trivial things (obviously the engine itself 2d focused for instance, and i shouldn’t be designing an FPS in it). Otherwise you run into exactly these kinds of issues (and eventually it turns into unity syndrome)