Yes, you can include the username and password in the url. Use the following url but replace username and password with your IceTV username and password.
http://iceguide.icetv.com.au/cgi-bin/epg/iceguide.cgi?op=xmlguide -u username -p password
EDIT That may not work in Emby as those parameters are for use with our webget.exe downloader.
If Emby supports running a program or batch file to download the xmltv file then that could definitely be achieved.