Assign case role isn't a choice in the search filters list in the activities table at the bottom of the manage case screen

See for example any manage case screen on drupal.demo.civicrm.org.

I think what's happening is that in general the list of activities in dropdowns is filtered by case type, which is controlled by the xml file. So it could be made to appear by adding it to the xml file, but then this would also allow you to create an activity of that type via the dropdown near the top of the manage case screen, but the appropriate fields don't appear on the activity form and the assignment stuff behind the scenes doesn't get done.

Maybe simplest is to just allow all case activity types to appear in the dropdown list for the search filter.