Does horror marking really effect non-commander units? Sure, if you happen to be in the very rare battle that somebody is summoning horrors without first horror marking you I suppose the horrors will attack you, but this seems like a very fringe case to worry about. Independent horror attacks only happen on commanders, right? What I'm getting at is it might be worthwhile to put a castle on such a site and man it with commanderless troops. Enemy will have to siege it with obvious consequences. After they've been sieging for a couple turns attack them with some guys casting hell power, summon horror, summon lashimas, etc.
This would work well with undead on one of the disease sites to (they wouldn't even need to be commanderless). Not sure if the effect is worth that much trouble, but if you're using undead anyway...