Hi @M42,
thank you for uploading the example image it helped me a lot and I got very good news for you: 
That is the result of the attached workflow, the trick was to to use Flowvariables to set the desired region in the Image Cropper node.
I feel you, the documentation needs a lot of love at the moment.
PS: here is the workflow: Autocropping-Gabriel-Final.knwf (207.1 KB)
best,
Gabriel