So you have non-ORM logic in your ORM model? I haven't dug into the Core to see what the differences between extending an ORM model vs. a normal model are so I guess I always assumed it would have caused problems.
How it gets the data, I couldn't care less (from a controller point of view). It also allows you to swap your ORM and/or DB store by something else, and your controller would be oblivious...
As long as it's not too complex, there is no problem if you do that in the same model class. If it gets more complex, or if the methods are not directly related to a single model, I use separate model classes, which in turn use their underlying ORM models.