I think it might as well be embedded into the Jazz2 executable. The client uses a random port for the UDP connection (can be explicitly chosen using BlurredD's patch though), and it would be quite messy to determine which port was used in order to forward it. An alternative would be to use both BlurredD's patch and the standalone port-forwarding program.

Also, I might be wrong, but it seems that the routers that support UPnP are the more "modern" ones, which would already be able to run JJ2 without any port forwarding at all. In general, I think the server would be more likely to benefit from this implementation, since the server TCP port could be forwarded automatically.