Assign field permission to individual users? Here the idea is to avoid defining dozens of roles to account for delicate permission requirements of users. 

Say, on a certain node that has 100 fields, a webmaster may eventually need to define several hundreds of permission sets, many of them are needed for very few users on and ad hoc manner. Being able to define field permissions for particular users gives the webmaster ultimate flexibility and avoids insane proliferation of roles. 

I appreciate your hints about this possibility.
You can manage this with #access if you're using hook_form_alter to make and manage the fields. 


  Description: Whether the element is accessible or not; when FALSE, the
  element is not rendered and the user submitted value is not taken into
  consideration.


Another idea is get help of access callback in hook_menu 


  "access callback": A function returning TRUE if the user has access
  rights to this menu item, and FALSE if not. It can also be a boolean
  constant instead of a function, and you can also use numeric values
  (will be cast to boolean). Defaults to user_access() unless a value is
  inherited from the parent menu item; only MENU_DEFAULT_LOCAL_TASK
  items can inherit access callbacks. To use the user_access() default
  callback, you must specify the permission to check as 'access
  arguments'


The idea is to make a function that resolves permissions for individual user for certain pages.