With the newest grid ( WordPress version 1.8.1 and up) there is a new type of boxes available.

With configuration boxes you can inject configuration to containers and slots. The boxes are not visible in grid editor per default because they need additional configuration to work.

To use container- or slot configuration boxes you have to implement the configuration_box_alter_content_structure hook and define a content structure for your configuration. Have a look at editor-widgets for the available structure elements.

WordPress

add_filter('grid_configuration_box_alter_content_structure', 
function($cs, $type){
  if($type == "container_configuration"){
    $cs[] = array(
      "key"   => "varname",
      "label" => "My label",
      "type"  => "text",
    );
  }
  if($type == "slot_configuration"){
    // TODO: add content structure items to $cs array
  }
  return $cs;
});