How about using iptables? I just checked, and there is a 'man' page on it on my N800 running OS2008. I do not think there is a GUI front end ported, that I know of. But it can be configured from the command line.

EDIT - I just thought about your situation further. If you are on a network that is not yours, then web traffic can only go to your tablet's http server if someone on that network types in your DHCP reserved IP into their web browser. They cannot access your server's information if they know the URL, since the network that you are connected to cannot direct web traffic to your server.