Well, thats not too hard to tack on. Just make it so that if Celest helps the elemental priest, he will convince some of his followers to convert to the Light, and if Magnagora helps the elemental priest, some will become enamoured with Taint. Sort of like a long-term no-real-effect village that can be influenced to one side or the other. If all of the followers turn to a particular side, there is some kind of bonus for that city.

But anyway, Lisaera has pointed out that this idea wouldn't be acceptable, so it doesn't matter too much.