Pseudo-field for custom entity without hook
As time goes by there's Extra Field now.


  Provides a plugin type for extra fields in entity view.
  
  This module allows developers to add custom fields by simply providing
  a plugin. Site builders can use these fields in entity view modes as
  normally. Extra fields do not store data and do not have a field
  widget. Extra fields typically combine existing entity data and format
  it for display.