ioq3 has guid support but it seems only be used in some mod for admin purpose.

But it could also be useful for server statictics purpose to retrieve the player GUID.
I know VSP could handle GUID but not for Q3 at that time. That product is quite dead now and I will try to produce a dedicated parser for westernq3/Smokin'Guns mod with GUID support (maybe I could also release an updated Q3 parser) as I still proposed a patch for their next SA release (see http://www.smokin-guns.net/main/viewtopic.php?p=8940).

Here is the proposed patch upon trunk rev1271 (but concerned file is rev1204):
--- g_client.c.no_guid  2008-03-14 12:15:49.000000000 +0100
+++ g_client.c  2008-03-14 12:17:11.000000000 +0100
@@ -713,6 +713,7 @@
        char    redTeam[MAX_INFO_STRING];
        char    blueTeam[MAX_INFO_STRING];
        char    userinfo[MAX_INFO_STRING];
+       char    guid[MAX_INFO_STRING];
 
        ent = g_entities + clientNum;
        client = ent->client;
@@ -863,6 +864,7 @@
 
        strcpy(redTeam, Info_ValueForKey( userinfo, "g_redteam" ));
        strcpy(blueTeam, Info_ValueForKey( userinfo, "g_blueteam" ));
+       strcpy(guid, Info_ValueForKey( userinfo, "cl_guid" ));
 
        // send over a subset of the userinfo keys so other clients can
        // print scoreboards, display models, and play custom sounds
@@ -871,10 +873,14 @@
                        client->pers.netname, team, model, headModel, c1, c2,
                        client->pers.maxHealth, client->sess.wins, client->sess.losses,
                        Info_ValueForKey( userinfo, "skill" ), teamTask, teamLeader );
-       } else {
+       } else if (*guid == 0) {
                s = va("n\\%s\\t\\%i\\model\\%s\\hmodel\\%s\\g_redteam\\%s\\g_blueteam\\%s\\c1\\%s\\c2\\%s\\hc\\%i\\w\\%i\\l\\%i\\tt\\%d\\tl\\%d",
                        client->pers.netname, client->sess.sessionTeam, model, headModel, redTeam, blueTeam, c1, c2,
                        client->pers.maxHealth, client->sess.wins, client->sess.losses, teamTask, teamLeader);
+       } else {
+               s = va("guid\\%s\\n\\%s\\t\\%i\\model\\%s\\hmodel\\%s\\g_redteam\\%s\\g_blueteam\\%s\\c1\\%s\\c2\\%s\\hc\\%i\\w\\%i\\l\\%i\\tt\\%d\\tl\\%d",
+                       guid, client->pers.netname, client->sess.sessionTeam, model, headModel, redTeam, blueTeam, c1, c2,
+                       client->pers.maxHealth, client->sess.wins, client->sess.losses, teamTask, teamLeader);
        }
 
        trap_SetConfigstring( CS_PLAYERS+clientNum, s );