There might be a way to restrict some spells to specific AI factions, and if so, you could make it so that the computer gets a 'Summon Champion X' spell when it researches some appropriately placed technology. I am not sure that this is possible, however, and even if it is, I wouldn't know how to do it.

Great, thanks! I've also assigned 8000 fame to this faction on start, so the behavior looks just like they are selecting faction-specific champions instead of drawing from the general pool. It should even be possible to link this this back to the fame resource, so that the AI will cast these spells at 8003, 8050, 8100, etc. But either way it's working the way I want it to, thanks for the help!