There is now API you can use to add items to the Drop Bar in Dropzone, unfortunately this currently has to be called from within a Dropzone action and can't be called externally by another app (such as Hazel). The best you could do at the moment would be clicking an action in the grid that adds the image from the folder or clipboard into Drop Bar, but that kind of defeats the point since you would obviously want the screenshot to be added into Dropzone automatically.
I think the real solution here is that Dropzone needs a way to run actions via a Terminal command. You could then have Dropzone periodically run an action to take the screenshot from the folder and then add it to Drop Bar. I'm planning to add this as a feature in an upcoming update.
