ojhwel-
I am looking into this for a client as well. Did you ever come up with a solution?
This isn’t too difficult, the folder holding those classes is located in a /js folder within the wp-includes/. I would be a little more specific but I’m not at my computer right now. You’re looking for the TinyMCE folder. Give that a go and if you haven’t found it by tomorrow give me a shout.
http://www.doc4design.com
Instead of attacking the core try this. Download TinyMCE Advanced: http://wordpress.org/extend/plugins/tinymce-advanced/ upload and activate the plugin. Within this plugins folder: tinymce-advanced/css/tadv-mce.css add your styles here. You may need to refresh the post edit page several times and clear the browser cache to see the changes.
http://www.doc4design.com
