My 770 is running the standard OS2006 edition, which doesn't support NAT out of the box.
Before I go through the hassle of compiling a custom kernel:

Has anybody already built working modules that enable NAT with iptables? If not, is there a way to 'emulate' NAT with the existing iptables?

There is a project providing NAT for iptables at garage.maemo.org. But I don't remember it's name and I couldn't get these modules to work. Maybe I did something wrong or missed something.

I think you were referring to this. I gave it a quick try but also didn't get it to work, since there is no (?) documentation for the modules.