I've been working on solving this same problem, slightly different context, but I've been unable so far to figure out how to make the feedback module communicate with the gradebook. If you view the forum topic I've started, and start with the 6th post into that discussion, you can see what else has been suggested and given as input.
Restricting Posts to Certain People
and I've started a new post with that as the title and haven't even gotten a single reply there yet..
Feedback Activity NOT GRADABLE
Feedback
Feedback,evaluations, and grading...
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