[eluser]Mirage[/eluser]
The problem you're running into is simply a [conceivable] limitation of a pre-fab framework like CI. In a glue type framework like Zend you'd automatically 'include' your libraries to get access to the functionality. With hooks, especially the ones that execute very early in the system, you have to let go of the Loader type calls because much of what's it depends on may not have been loaded yet.
Only count on what's available at a given point. Beyond that, there's nothing wrong with using php 'includes' to load your [CI] resources and instantiate them yourself.
That said - for Auth/ACL specific stuff I've created a _PrivilegedController.php which does what needs doing in it's constructor. I subclass my controllers from there and presto! Instant soup! No need to hack CI for this one.
It would be nice if in some subsequent release of CI the core would actually be re-written so that you can extend the CI controller the same way that you extend other libraries. The current desgin doesn't allow for this, but it's definitely possible.
Cheerio!