I'm now Adored in Yew, Minoc, Moonglow and Skara Brae. I can confirm the following;
* There are only ever 2 "free" Rioters at any time. The trigger for a new one is arresting one; I can confirm that one as I got very lucky and saw that as soon as I used the rope on one, another spawned in the same room behind me. I don't believe now that it's possible to grief the event by sitting on an arrested Rioter then.
* If you can't see any, they will still have spawned somewhere; but some of the cities have rather a strange definition of spawn locations. For Yew, it includes all the way down south to the last farming hut, inside Empath Abbey, and also the cells of Yew Prison, including in the walls. You need to approach from outside to get at those, as the cell doors are now locked. For Moonglow, they can appear in any of the NPC farms on the entire island, as well as the Lyceum and the woods immediately around the city.
* My hope is that if there is Venerated, it's the final level of reputation across the entire arc, and that we're trying to rush to the end before 7 more months of content. Because the idea of having to grind even one character to Venerated now, on rioters alone, especially in Yew is appalling. I don't mind coming back and polishing off to the final level with other services to the realm, but that would be way too much. And I wish the Devs would stop adding ulta-hit point, ultra-grindy, OCD-encouraging loot and quests and arcs; Ultima Online is a Sandbox, not a Diablo/World of Warcraft game, which is why I still play it and neither of those two anymore.