We have many legacy systems that span databases with SSN or other unique sensitive info about someone. Developers and testers needed a way to compare a paticular person and test across systems so the values needed to be identical. Each person has a unique employee or person id, then I run a script against all the databases and tables to update the SSN, and other numeric unique sensitive fields based on SET [SSN] = '100000000'+[EmployeeID]

We don't have a massive amount of data so this worked very well for us and very simple to

manage since everyone has a unique id the math above will generate the same ssn or other unique value everytime unless their id changes, which it never does for us.