Hi -
This may or may not be what you are looking for -- I did talk briefly about grading StoryMaps. The University of Minnesota has some sample rubrics available for StoryMaps that focus more on content, less on cartography, since the classes are not about cartography. Please note that this site is being revamped, and more content is coming.
Another panelist shared this document, Map Evaluation Guidelines, that is more about the map. I plan to incorporate this into the UMN StoryMaps site.