HI Ben,
You seem like you may have this solved, but there are three things that come to mind in addition to what you and Bastiaan discussed.
(1) Disabling stage logging will work unless someone overrides the logging level from the System Tab > Resources > Management area. If you right click on a Resource, you'll see that there are logging level overrides there. Just something to consider. This might just mean you should restrict access to that part of the System to only people who understand how to use that section.
(2) In addition to disabling stage logging, you should tick the checkbox in action stages where it says ""Don't log parameters on this stage"".
(3) You could also treat the SSN as a password datatype. It will work just fine for data entry where necessary, but if it does happen to get logged anywhere such as session logs, etc., it will not show the actual SSN. (actually a 4th thing comes to mind now that you could encrypt the SSN when you first retrieve it and then decrypt it before you write it anywhere. But that might be overkill. Haha)
Anyway, just some input. Hope it helps!
Respectfully,
Dave
Dave Morris, 3Ci at Southern Company