This isn't possible with the ORM class. You can build your own query with Kohana's own query builder and the object can be returned as an ORM-model, therefore you fill in your ORM model name (e.g. 'my_orm_model') in the as_object function.
Combining one of your suggested links with Kohana's own Query builder you would get something like this.
DB::Query(Database::SELECT,"SELECT LEFT(field, 40) AS excerpt FROM table(s) WHERE ...")->as_object('my_orm_class')->execute();