How to mask all but last 4 digits of SSN on People form (EBS 12.1.3) for a specific responsibility

Requirement:   Mask all but the last 4 digits of the SSN (National Identifier) on the People form for a specific responsibility?

I can use the "CONCEAL_DATA" property to mask the entire SSN for a specified responsibility .... but we want the last 4 digits to display.  

I tried to use "FORMAT_MASK" but it isn't working.   I found note 1210843.1 that states "Format Mask" can be applied only to NUMBER and DATE type fields.     

Questions:

1) Can FORMAT_MASK be used on the PERSON_NATIONAL_IDENTIFIER target via Form Personalizations?

2) If not, is there another way in Form Personalizations to mask the PERSON_NATIONAL_IDENTIFIER?