Does you know how the CCK Private Fields module works? Any API?
Looking at the description page of this module, it seems that user can do what you want:


  Overview:
  
  This module allows users to mark administrator selected CCK fields as
  Public (visible to everyone), Hidden (visible to node author, hidden
  from everyone else) or Private (visible to node author and "friends";
  see requirements section below).


Regarding the option to "disappear" a user, I'm not really sure as there's a lot of place where a user can appear, however if you give this user a certain role, you can customize your views to exclude users with this role.