Well, i think the trigger crate would be in the UDP portion of the jj2 network. TCP deals with chatting, downloading levels, and downloading the list of people playing; UDP deals with the actual gameplay.

I know of a program called WPE (windows packet editor) that could intercept packets and either view them, edit them, or add more packets. So it could be possible via that route, but i'm not sure how. Another way would be to create some sort of proxy that would transfer data back and forth from the server, and when need be, send the trigger data. As far as i know, you might have to use two different computers for the proxy method because you can't change what port jj2 communicates on, thus you couldn't run the server and the proxy on the same computer (they'd both use the same port). And a third option would be to create a full-functional server and add in the option to send the packet request. This would be by far the most versatile, but by far the hardest.

-cheeze