Network Connect stores its previous connection information at the following location for Windows platform:
C:\Documents and Settings\<username>\Application Data\Juniper Networks\Network Connect 6.1.0\versionInfo.ini
(substitue the proper version of your NC install in the path)
I was able to manipulate the versionInfo.ini file (parameter URL_0) to prepopulate the required URL to use for logging in. Hope this helps.