You can't. If you don't have permission to open the work item, then you can't query information about it. It's a bit of chicken/egg. Even when you do have permission, you cannot be sure a workitem was deleted, or has not yet been created.
I'd run your tool as a service somewhere under a user that has project collection level permission or server level permissions to read the work items. You could also implement your own webservice which does that and then use the outcome in your tool.