Okay, I have been messing around with this for the last hour and finally narrowed down the problem. There is a setting in the configure->toggles section called "improve UDP/IP networking quality" that was apparently only added in one of the newer builds and is on by default. I'm not sure what it does, but it seems to be the source of all the trouble. Once it's disabled, the games suddenly become visible. Both the host and the players need to have it turned off.
I have put up the old build you sent me here
in case we still run into trouble, but let's give the latest one (1.5.173) another try tonight, with that setting turned off.
There is one other strange problem I ran into in the newer build, where if I joined a game, left it, didn't close D2X-XL and rejoined the game, I kept getting destroyed instantly whenever I tried to move. So if that happens to you, just try closing down the game and reopening it (this also applies to the host). I was also earlier running into some crashes when loading up certain savegames, but they seemed to go away when I got rid of my pilot file and created a fresh new one in its place.
Also, it looks like my games are now visible, so I should be able to host if needed.