Ha, I looked through "modref" thinking I was sure that existed somewhere, but forgot to check "ashref".
There's something I need to know before I start to implement a partial stagger success chance: when a stagger is ignored, does that mean all effects of the item are ignored (e.g. damage, deleveling), or just the stagger effect? For example, a disease has a base 50% stagger chance, in addition to some deleveling. A monster with a 50% chance to ignore staggers will only be staggered by a disease 25% of the time, but will the deleveling always occur? Please say yes.
While I'm at it, I'm also going to take this opportunity to rejigger BatBrain to follow KoL's terminology (i.e. "nostagger" rather than "nostun").
EDIT: r41 converts the old boolean "nostun" to a float "nostagger", allowing us to support the progressive chance to ignore staggers granted by +ML. Terminology was likewise changed in batfactors, though the old "nostun" language is deprecated but still supported just in case.