I have used process given at link http://community.bonitasoft.com/project/process-variable-example.
When i Run and complete process and checked archived Cases Tab, There was no Case variable value in Archived cases. Case Variable value was visible in Open cases during process workflow but not in archived cases when process finished.
This is currently a know limitation. I'm quoting the documentation of the community example:
Overview form is not able to display process variable value for an archived process instance (lack of REST API).