I am trying out GhostDoc, StyleCop, CodeIt.Right, FXCop etc to find a streamlined way of adding both autogenerated XML-Comments in C# code as well as updating existing comments in case parameters changed for example.
GhostDoc Pro seems to work well, except for the fact that I cant get a report on coverage, or at least the classes that completely lack documentation.
How can I get such a report?
Like you mentioned, GhostDoc will help you with this; you just may need a newer version or the right edition.
You can use the GhostDoc Pro Documentation Maintenance feature in the v5 - it will help you find auto-generated docs, find classes/methods that are missing documentation, as well as those that require updating (method signature out of sync) or editing. Here is more info on the v5 features - http://submain.com/blog/ReleasedGhostDocV50.aspx
You mentioned you also use CodeIt.Right - you want to turn On rule GE00005 - Externally visible types and members should have XML comments (Type, Member -> Should have XML comments). You can also export the list in VS or generate it in your build.
Both GhostDoc and CodeIt.Right will generate XML comment templates for the missing docs but, of course, you'd want to edit them afterward.