Currently in Joomla hooks can't be used because there is no invocation of civicrm plugins that someone might create. This code will invoke civicrm plugins

Currently CiviCRM does not take advantage of hooks in Joomla because it does not invoke plugins.

Addint this code:

//invoke plugins.
JPluginHelper::importPlugin('civicrm');
$app =& JFactory::getApplication();
$app->triggerEvent('onCiviLoad');

near the top of admin.civicrm.php will allow access to a potential set of civicrm plugins that a user might create or that might be included in a release.

And example of a plugin is attached.