"What if bless armour stuck to adding protection, but that protection was based on where the damage was coming from rather than a static amount?"
If an undead hits a body part which has an item with blessed armour, there is a certain chance of a holy critback...? Only versus undeads, nothing else.
Oh, and if an undead (lich or ghoul) wears armour that's blessed, there is a small chance each round of them being holy critted.
That would be fun.
Similarly... I always thought it'd be fun if Clerics had a True Weapon like spell. Not stackable with True Weapon, and only having the True-effect on undeads and followers of Tobin/Marghuul. To differentiate further from True Weapon, maybe give it some further abilities... make it increase holy crits +1 more than True Weapon.
It would allow for some increased Weapon variety, even if most would just prefer True Weapon, Paladins and War-priests especially would want a... Holy Avenger (Cliché name, I know...) weapon for special circumstances.
Something like that