So far I am very pleased so far with this class because it has thus far been more true to what a ninja is than any other version I have ever seen from 2nd to 3.5. However, I was just re-watching an episode of Deadliest Warrior and I was reminded of the Ninja's black egg. I am sure this is something you could house rule but still it would be neat for something official.
Black Eggs where hollowed out eggs, filled with finely ground glass and other substances such as liquid chilli juice; this would then be throw at the enemies faces blinding them with glass under their eyelids or stinging their eyes with the first pepper spray.
In game terms I could see this blinding the foe with a ranged touch attack for a number of rounds thus leaving them open to sneak attacks.
I see metsubushi happō as mundane items that grant a one-time bonus on the Dirty-Trick maneuver.