Ahh yes, it would be possible to have any new games use the SIGS protocol to find and connect to games. That can work quite nicely. I guess if I ever get this going, I'll be providing details and/or code to help with that.
And yeah, Outpost2 has it's own network protocol and SIGS uses another. Now, SIGS doesn't have a problem with NAT because it uses only outgoing connections (and TCP which is inherently bidirectinoal, so by default all data from the server comes back along the same connection). This is important since your routers that do NAT are seperate from your machine, and your machine has no idea what they're doing. They essentially work by extending the address space for IP addresses by using the port field. To do this, they need to modify both the IP and the port (source) of outgoing packets. If it receives a reply to one of these packets, it can look it up in it's tables to figure out where the initial data came from and where it needs to send replies back to. Now, if you're running some kind of server behind a router, there is no initial data being sent out so the router never gets a chance to build it's tables. If it receives an incomming packet, it doesn't know which internal IP to send it to. Which is why you have to configure port forwarding in your router, so it knows where to send things by default. The problem with OP2, is that it essentially treats every client in a server like fashion where it listens in on some fixed port for incomming data, but has never sent data out on that port. Also, OP2 doesn't reply using the source address on the packet, but instead replies to a fixed port number, so even if the other client had sent data out on some port, it doesn't reply with the correct (translated) port to make it back through the router.
I hope that helps. Anyways, SIGS just gives the IP, and maybe the port number of the host to the people who want to join. It would be entirely possible for the server to keep track of the source ports on the incomming packets, but that only gets you so far. There are three main ports that OP2 uses, in some order, and this could likely only get you through the first set of connections.
Technically OP2 isn't to blame since they've followed IP standards in their design. It's NAT that wasn't built according to the standard. Hence why it screws up so much. But then, I guess most applications work according to some unwritten standard that NAT made use of.