I have few questions...

[eluser]TheFuzzy0ne[/eluser]
Yes, although it's not quite as easy. Just put your callback into a helper, model, library or whereever else you want, and then you can simply reference that call back from your controller with an alias.

Code:

function _my_callback($str="")
{
    return $this->my_validation_model->my_callback($str);
}

I think you can also access it in a model/library directly by defining you're rule something like this:

Code:

...
'rules' => 'callback_my_model->some_method',
...