This probably isn’t the most elegant solution you could add a new DB text field to hold the css icon class for each page. Then you can output the $mycssicon inside the navigation markup wherever you need.
It’ll work but as I say might not be the most elegant solution.
You could also use something that already exists on SiteTree for everything in the navigation…
Or create your own custom method on your controller to take $this->URLSegment, lowercase it and strip the dashes and return it to your template if you wanted to.