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Select lists are difficult to work with on multiselect fields.
I just figured out how to do this by creating a list and making that the access moderation field. I think the default would be nice if you could change the widget too.
If I understand this correctly, you can already have a taxonomy term that uses Check boxes/radio buttons and use that as the access moderation field.
I would like to see how you could add access control using different input widgets like https://drupal.org/project/multiple_selects to work with Workbench Access.
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