Sql server covering index and join
Not sure what you mean by:


  the join would force the clustered key to be matched.


Non-clustered indexes include the clustered index key (needed as the unique row locator).  Consequently, a non-clustered index can cover a query as long as the query references only columns in the clustered index key, non-clustered index key columns, and included columns.  

A potentially covering non-clustered index might not be used if other indexes are available to process the query more efficiently depending on the columns specified in WHERE, JOIN, or ORDER BY clauses.