I'm sorry, but I'm either a bit slow or I'm misunderstanding you. So what if there are multiple IP addresses? The important thing is that people use the same hostname to reach the forums. forum.domain.com could be setup in DNS to point to e.g. 213.10.55.10. This is the address people use to access the forums from the Internet. For people visiting the forums locally or via the intranet, their hosts file could override this DNS entry pointing it to e.g. 192.160.10.155 or 127.0.0.1.
Also, you are aware of the fact that the referrer check only applies to admins and moderators, right? Only a fraction of the people using the forums should be affected.
"Programming is like sex: one mistake and you have to support it for the rest of your life."