Zero points for Essay keywords

I am using the Essay content type in an activity for practicing paraphrase and summary. At present, I can only give automated feedback for keywords that score at least point. However, I would also like to automate feedback for keywords that score zero points.

The purpose of this activity is to have students think for themselves. However, students are prone to copying from examples. At present, I cannot discourage this with automated feedback, because all keywords award points. If we could give zero points, it would be possible, for example, to include an entire sentence or phrase as a keyword, award zero points, and have the automated feedback read "This sentence was copied directly from the example."

With a value of zero for a keyword, we could also control for nonsensical or incorrect answers and direct students to find the expected information. These could also work as hints, where the student first enters an answer and then the automated feedback directs to a better answer. The better answer then is awarded points.

A minimum point value of zero would enable more creative uses of automated feedback for items that cannot be awared points.