You can not start UT2004 from xchat with UT2004-links (ut2004://) normally (normal IP works). To make those links clickable you have to compile xchat from source.
Download the source and decompress it. Open the file: /xchat2/src/common/url.c with a text editor. Find the line { D("gopher://") }, .Copy that line and paste it under the line you just copied. Change gopher:// to ut2004://. When you are done it looks like this:
----Cut-----
{ D("rtsp://") },
{ D("gopher://") },
{ D("ut2004://") },
},
suffix[] = {
{ D(".org") },
{ D(".net") },
----cut----
Save the file and compile. When xchat is installed, start xchat and go to settings --> advanced --> URL-Handler. Make a new post and name it Open with UT2004 (or whatever you prefer) then add the command !ut2004 %s Make a new post and Name it Spec with UT2004 and the command !ut2004 %s?SpectatorOnly=1 Now you can start UT2004 as a player or as a spectator from Xchat if you right click on either a "normal IP" or a ut2004://IP
I expect that this will work for UT3 when that time comes, but the URL will probably be ut3://