Unique Workbook Identifier

Here at work we have a set of 12 workbooks, that form an extensive activity plan. I have created an add-in that creates a worksheet menu bar item and adds different functionalities to these 12 files. Because any user might have more than one of these documents open at one time, I am forced to refer to the documents by ActiveWorkbook. A bonus added feature: the names of these files change periodically, but not at the same time.

Before running any procedure I must validate that the active book is one of these files. Currently I am testing the existence of a veryhidden sheet codenamed "shIsValid" in the workbook. Now I'm thinking, there has to be a better way to do this. Simplest way would be if ThisWorkbook object had a Tag property similar to UserForms, but this doesn't seem to be the case. I wonder, is there actually a way to give a workbook any identifier (boolean, string or number) that I can access directly, so I can get rid of these dummy worksheets.