StarRating questions are better addressed to the extension forum http://projects.ez.no/index.php//starrating/forum
Once you have access to the content attribute you can access the average via $attribute.content.rounded_average or $attribute.content.average
Retrieving the average across all content classes is a little more difficult. Each rating is stored in the database http://svn.projects.ez.no/starrating/sql/starrating.sql Once you know the attribute id ( contentobject_attribute_id ) of the content object it shouldn't be hard to work out the average.
If I have time I'll look at adding this to the extension.