I see now, i dont think there would be any other work around if you cant install the required scripts to allow iis proper understanding of x-forwarded-for

You could re-write the stack and swap remote_addr for x_forwarded_for but that would probably break more things and its not something that is easily done.

im assuming you require x-forward for proper session/user management? if so, thought about using phpsessid or something similar then match on the unique cookie and handle the connection properly?

If the issue is more of user authentication via real ip then you could use LDAP with nginx and try to authenticate users that way