To answer his question: No. Throwing a razor sharp boomerang knife is not a good way to incapacitate someone. However, from my boomerang throwing experience I think that a normal, non-knife-like boomerang could do the job. Batman would then need a separate shuriken weapon for quick cutting but nonlethal pain.
Building Batman will be training with shuriken next month. ' Can't wait.
P.S. Disney bought Marvel this morning (assuming the investors ok the 4 billion dollar deal).