instead, if you can PPTP to that machine and NAT/masquerade behind it, you can portforward the game to it. My personal record for SFC online gameplay at once was 4 players.
1- local portforard
2- portforard through neighbour. (he had a networking link to there too)
3- PPTP + forwarding
4- something that I wrote.. "PPP over SSH" which allows me, like PPTP, to get a remote IP using between 2 linux boxes.