I just chanced on this thread.
It may be helpful to note that SAP Custom Development has released an add-on product, UI Masking / (a.k.a. UI Field Security), a generic functionality to achieve data masking for sensitive fields through configuration. Currently works for SAP GUI; other UI channels are being considered.
More information can be found on the following SCN page: http://scn.sap.com/docs/DOC-57739