Category Visibility & Levels

Just wondering if anyone has ever had a go at trying to add a level hack to Gudlyf’s Category Visibility plugin.
http://www.gudlyf.com/archives/2005/03/08/wordpress-plugin-category-visibility/

Is it something that could be achieved by only modifying the plugin or would the actual wp files need to be hacked too?

Unfortunately it is currently beyond my skill. But am happy to try if someone can set me on the right track.

I presume it is just a question of wrapping an if statement around something or other…