It should be posted in the CGI variables available to your web server (this will require some sort of web programmer to be able to extract this data, since it normally isn't stored in the standard web logs. If so, and you setup your own web server, you can actually pull out the serial number out of the HTTP request when the phone pulls a copy of the 96xxupgrade.txt file. Are you in a position where you can have all the phones reboot?