Dear Charlie,
If I get your right, you are putting up another webserver behind the SME gateway and proxypass might be your answer.
For my case, my router with inbuilt firewall is my gateway and my SME host mail and website. I have only one fixed IP and required to host a web application on a redhat box. To solve my problem, I installed proxypass on SME and it direct the traffic to the redhat web application internally and externally.
Just wonder is there other better solution for my case.
Do a search for proxypass. Hope that help.
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