Boy, it's amazing how many people have asked the same question: how to
forward an internal webserver as if it were on a 2.4 Kernel firewall using
iptables.  You'll get pages of hits on Google's usenet archives.  

Very few of the queries have answers.  Mostly the answers are "like, I'm
still running Windows, but I think you need ipchains for that".

I'm running a firewall script derived from David Ranch's Trinity OS Project,
and I've written him to see if he's willing to provide an example script.
Who knows?  A question this big deserves an answer.