Although I was not able to definitively determine how and when exactly Zahal the Vile spawns, I was able to rule a few things out after 8 hours camping him during classic on the Oakwynd server.
1. The Silver Griffon does not appear to be a placeholder. I tracked the zone most of the day and never saw The Silver Griffon, although Zahal the Vile spawned 4 times. Others have reported they have seen both mobs up at the same time. Either way, I don't think The Silver Griffon as a PH for Zahal is verifiable.
2. Killing at Undead camp at loc -3200, -2100 had no correlation with his spawning.
3. His spawn time was approximately every 2 hours. He spawned 4 times in an 8 hour period.
4. It was difficult to tell where he spawned, as he was killed rapidly after spawning, by myself or someone else. The Bracers of the Reverent quest (https://everquest.allakhazam.com/db/quest.html?quest=245) lists a spawn point of -1800, 200 (down the hill from the Guard Tower). I sat here for hours and never saw him spawn there. Brewall's map marks him northeast of the gypsy camp, and I can say he was killed near this location twice. Two more times, he was killed further southeast, toward the farm. I'm not sure if something changed, but it appears he paths now. Another player reported that he spawned on the west side of the zone, but I was unable to substantiate this.
5. He spawns as a troll, not a ghoul, but sense undead still works to track him. I believe he was changed to a troll in order to drop Troll Guts for the Fabian's Strings quest https://everquest.allakhazam.com/db/quest.html?quest=549 in the PoP era. Him spawning as a ghoul is only for p99.