All of the URLs on the SA are static. You will need an external solution that understands IP source and URL redirection.
You can make a Realm restriction that will only allow a specific subnet to login to a URL, but the user will need to know the specific URL.
You could probably address this problem with different dns entries on the client sides.
So clients from network 1 which use dns server X get another ip for login.company.net then the users from network 2 who use dns server 2. So there could be two different IPs for the same name.
Wouldn't it be possible to use variables in the bookmarks just as you can do with <USERNAME> when providing a File Bookmark for a user home directory?
Only problem would be to find out what variables can be used in bookmarks. I haven't found any compilation of codes and variables available for different places in the config.