The key is to bind the mujina into something launchable by a siege engine. This should be strong enough to handle launching, but weak enough to break upon impact, releasing the mujina to wreak havoc among the target castle's inhabitants.
The use of such a weapon appears nowhere in our records; indeed, I must ask how the binding shugenja will guarantee the mujina won't just zip back to those who put it into it's position to begin with, rather than the intended targets. There is also the fact that the Crab, being responsible for defending the border with the Shadowlands, would find it relatively useless, unless they were to turn inward against the other clans -- an impossibility, though, as the Crab are the personification of Duty.
Return to the Intelligence Bureau.