Hi Brad,
You can implement the analytics code on the sign up page that comes as a modal pop up. This sign up page may be an html page, so you can implement the analytics variables at the page level and call the core js file (s_code.js).
You can use the jquery colorbox plug in.
At the page where you have the button/link to that modal window (the sign up page), just place this code just before </head> tag:
<link rel="stylesheet" href="colorbox.css" />
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
<script src="jquery.colorbox.js"></script>
<script>
$(document).ready(function(){
//Examples of how to assign the Colorbox event to elements
$(".iframe").colorbox({iframe:true, width:"80%", height:"80%"});
});
</script>
Then add the href code with iframe to call your "Sign Up" page (The Modal Window) anywhere in the <body> tag, like this:
<a class='iframe' href="signup.htm">Sign Up</a>
You can see an example using Google Chrome web browser at the following test site:
http://analyticsawesomeness.bugs3.com/
Look at the page source for the code and "jquery.colorbox.js" file.
Hope that helps. 
Regards,
Ryan