I've accomplished almost what I wanted for the week. The Stealth plug-in now responds to hearing and vision, and Actor, Class, Item, Weapon, Armor and States can all modify these checks (i.e. a Cloak of Invisibility or a Muffle spell, or heavy, clanky armor).
I can also set hearing and vision modifiers by Region ID, so an area of floor can be noisy or silent, and brightness figures in as well.
Individual events can be set to Patrol based on Region ID, using the "closest matching Region ID I haven't been to yet" logic.
I don't have a visible Stealth gauge yet, and I'm still confirming the Vision checks act like I want, but overall it's in good shape. I might put it on the MV plug-in repository in early December if all goes well (debugging takes longer than the initial coding).