I got it working late yesterday afternoon.
Basically you add one line to the main index page to call a hidden iframe. The iframe page has about 25 lines of code that gets the Windows user name and writes it to a cookie. Because of a peculiar issue with IE, the iframe page needs to be in a subdir. It's a long story; this explains it:
http://www.websina.com/bugzero/kb/browser-ie.html(http://www.websina.com/bugzero/kb/browser-ie.html)
Then in open.inc.php you read the cookie, and populate username/email in the HTML form. Total code for everything is about 30 lines, and very minor mods to existing code. My internet at work is very restricted. When I get home tonight I'll upload the code somewhere so you can download it.
PS: it works on Apache (Linux/Windows) but not on IIS.