You can run a proxy on your N800 such as Privoxy or a Socks proxy.

You can use port forwarding, NAT, or routing on your NIT. For that you'd need something such as routed or iptables.

Privoxy is, I believe, the most simple solution, but your streaming application must support this. You can find a howto to install and enable Privoxy on the Maemo Wiki. You must do additional configuration though: you must allow your client's IP to connect to the Privoxy, and the Privoxy must listen on the WiFi device and IP.

Be advised the NIT only has a limited I/O available. I wouldn't expect it to be able to relay DVDs; in general the throughput isn't higher than 4 or 5 MB/sec.

UPnP isn't included in this story yet but it'd make some things even easier.