Actually this is easy to do it by using the controller. For example you can simply do:
function product($parameter){
$this->my_test_parameter = $parameter;
$crud = new grocery_CRUD();
...
$crud->callback_add_field('dropdown_field_name',array($this,'_add_field_callback'));
...
$output = $crud->render();
}
And the callback:
function _add_field_callback($parameter){
//load db model
//call the result and return as dropdown input field with selected selection when value = $parameter
$value = !empty($this->my_test_parameter) ? $this->my_test_parameter : '';
...
//here you can also use the form_dropdown of codeigniter (http://ellislab.com/codeigniter/user-guide/helpers/form_helper.html)
}
I know that you are desperately looking forward for the default value for grocery CRUD so I added an issue to the github https://github.com/scoumbourdis/grocery-crud/issues/138 . This is will be a reminder that this thing has to be fixed.