currently this work (with AR as string builder):

$array['first_name'] = 'John';
$array['last_name'] = 'Doe';
$this->examplemodel->db_set('insert_date', 'NOW()', FALSE);
$this->examplemodel->new_record($array)->save();

And it is a bit hacky.

With IQ as SQL-builder:

$array['first_name'] = 'John';
$array['last_name'] = 'Doe';
$this->db->set('insert_date', 'NOW()', FALSE);
$this->examplemodel->new_record($array)->save();