Thanks for trying my plugin Bozz. I've looked into this before, it would be great if you could manually crop is right as you add the photo, but I didn't make any headway on it.
I haven't really found any good examples of anyone using that cropping tool on its own, let alone with the featured image functionality. I am thinking that to use that tool, you'd have to build your own custom image manager rather than using featured images.
Really the autocropping that the featured image does is just a last resort to keep the slideshow images the correct size and you always get better results prepping the images in an image editor first.
But there is actually a way to manually crop them in WordPress, you can edit the image after uploading it and size or crop it before adding it as a featured image. It actually works pretty well, but it is a bit awkward for the end user because it doesn't automatically load the image editor as part of the featured image workflow, you have to dig into it.