Parse through all the Controls in the designer and call TypeDescriptor.Refresh() on them. From inside your custom IDesigner implementation:

private void UpdateExtendedProperties() 
{ 
  IDesignerHost idh = GetService( typeof( IDesignerHost ) ) as IDesignerHost; 
  foreach ( Component comp in idh.Container.Components ) 
  { 
    // ignore the Form 
    if ( ( comp is Control ) && ( (comp is Form ) == false ) ) 
      TypeDescriptor.Refresh(comp as Control); 
  } 
}

Contributed from George Shepherd's Windows Forms FAQ