Select All?

[axyr]

Would it be possible to augment the gridfield header with a select all checkbox ?

[gavro]

It's present, but the "search icon" replaces the checkbox, as it's added as last column.
There's a commented line @ GridFieldCheckboxSelectComponent.php:

    public function augmentColumns($gridField, &$columns) {
//      array_unshift($columns, self::CHECKBOX_COLUMN);
        $columns[] = self::CHECKBOX_COLUMN;
    }

If you uncomment this line, and comment the current active line, you'll get what you want. Or you can change the order yourself using setDisplayFields on your GridField.