The case I was thinking about is something like, let's say there's a obviously mentally ill guy thratening to kill people. I'm talking hobo smeared with shit, reeking like all hell, tattered clothes, broken bottle in hand, screaming obscenities and frightening people with death threats in parking lots. You know, regulard tuesday evening in LA.

I think the police should be allowed to arrest this man. A DA might decide to file this new mental health charge against him. Assuming the crazy person doesn't accept the plea deal he would be offered, voluntary commitment in a mental health facility, this goes to trial. If the trial finds the defendant guilty of being insane, he is forcefully commited to said mental health facilty.

Are there issues? Sure. Is it somewhat ok because it's done via due process? Maybe. Is it better then letting potentially violent crazies in desperate need of mental health treatment run loose? Absolutely.

You can already be arrested for the things your describing, they're crimes. And if the judge feels it's appropriate sent to hospital. If that isn't happening it's because cities (understandably) cannot pay for the enforcement or treatment. I don't think we need any change in the law for this.