Assign field permission to individual users?
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.