The test version for XenWord 3.0.1.04 is installed on this site. A key line was added to the class-xenword-login-xfusers.php file, possibly allowing WP Super Cache plugin to be working properly.
If you would like to test caching on your site then please open the file class-xenword-login-xfusers.php and add a line of code below the wp_set_current_user($user_id) found on line 103. Make sure that the function wp_validate_auth_cookie replacement exists in the file. The line to add is
wp_validate_auth_cookie($cookie = '', $scheme = '');
If you decide to test this feature then please provide feedback in the XenWord forums.