Are you sure that isn't already an option.

In ProSeries Professional   if  you go to Tools>Options>General   there is Mask Sensitive Data at the bottom of box   two check boxes for masking data

Curious...for what purpose?

@taxes96786     Lots of reasons for wanting to mask SS# on the screen.   #1 would be nosy clients or curious clients that want to watch what you're doing.....   If I need to switch from parents return to childs return there is a small amount of time my screen shows my Homebase.   

Gottcha...fortunately I don't allow others to hover over my computer. If it is the client I am working on, there isn't a problem with watching what I am doing.