An alternative is to have the online plugin itself use NAT punch-through like what Raknet uses.
That sounds like the smartest thing to do, so nobody has to set anything up to play. I think that's what I was thinking of but didn't know the name. I think it's actually very important since a lot of gamers are behind routers which do NAT.
RakNet looks promising, and is free for indy games (revenue under 250k) but some restrictions ("Must display RakNet logo in your game's splash screen or credits", "Downloadable distribution"), and has NAT punchthrough. SDL_net might be a good alternative, but I can't find much information about it.