Much like putting an enchantment on Cold Iron, I think the prospect of enchantments based on Nullification are rather counterproductive. They don't make sense, and yes I know this is a game about elves and pixies fighting dragons while dwarves arm wrestle eldritch horrors, and I invoked the Make Sense clause.
Their entire deal is taking away, so something like that goes against the grain. Also, if you don't have to have Nullblade levels to have a Nullifying weapon, then things get stupid crazy, stupid fast. Unless you add in the "You must have nullblade levels" in which case everyone just dabbles in Nuller, takes a nullifying voidstriking repeating crossbow, and erases your entire campaign.
Though, in light of this, perhaps a Dispel Magic type effect where a Nullblade can disable magical items for a while, like say... a few rounds per level or something? Now, since I'm at work and can't re-read this in depth, if this is already a class feature, then it's a good thing you listened to Past!Me when I time traveled back to inspire you.