Assuming you're using Google Analytics new async code, all you need to do is place this code in the JavaScript block where you render your modal dialog.
_gaq.push(['_trackPageview', '/fake/modal/path/here']);
If you're using the old code,
pageTracker._trackPageview('/fake/modal/path/here');
The basic gist of this is you should attach the _trackPageview call anywhere you're binding an event event that changes the modal. If you're doing this for multiple modal pageviews, you'll need to either manually code each internal modal page, or programmatically name them. If these modal changes trigger changes in the URL's hash, you could try this approach: http://stackoverflow.com/questions/3564733