That could be a good substitute. I prefer having a passive effect, because it seems obvious to me that religion should have such effects but, if Tom doesn't go for it, I'd be fine having it as a priest action. It'd give priests a way to harass infiltrators.
Err... actually, abuse alert: priests would become key anti-infil defenses. Find an infil, fast travel to his location, mob him with peasants. Now admittedly that seems somewhat plausible from a medieval perspective (foreign assassin gets caught up in backlash by peasant zealots when a priest points out who he is), but from a game balance perspective, infils have been pretty nerfed it seems to me, not sure if we want to do so even more.
More work then having a system where you can have a state religion, massive coverage in all your regions and just declare all known infiltrators working against you to be Anathema.
There are solutions, like requiring the noble to be a member of a religion that you consider evil, but things are just getting more complicated, and then you also have a incentive NOT to join a religion. You could also just give infils a bonus to avoiding such actions, since they are all stealthy and clever like.
The reason I suggested having a priest need to stir them up is that it seems implausible to me for peasants to regularly mob nobility without some sort of serious catalyst, an agitator to really stir up the existing ill feelings. You could just as easily have it as an action for any Elder member of the religion I guess, I picked Priest since they are typically good at rabble rousing so it seemed like a good RP fit and they already have a section for religion specific actions.
Without the agitator aspect to me it would seem that it should occur so rarely under a passive system that it is not really worth having.
