That sounds really cool actually, I'd like to see it
My guesses for why poison knives aren't in:
- bleeding from serrated knives already provides the damage over time aspect of poison if not the special effects
- AP cost, since knives' AP costs are 11-12, while poisoned throwing knives have an AP cost of 18, this would let you inflict poison at a much greater rate, the AP cost of poisoned knives could be increased however