I like the idea from experience
The road I live in has a mental health charity which has residents living in small rooms, they are free to come and go as they wish.
Mostly they spend their days keeping the local pub in business, I'm kinda ok with that but maybe the money they receive could be put to better use than beer.
However there was an incident a few weeks ago, someone was outside my house at 3am on a Sunday morning having an obvious mental breakdown, they were shouting and screaming at their mother (Who wasn't present) telling her that how much this person hated her. The person was going completely bersek at the top of their voice with awful foul language to match.
I was about to phone the police when other residents started coming out their houses telling this person to shut up and go home.
Now I have no idea if this person was from the mental health charity, but if they were tagged myslef and the charity would have been able to check to see if it was one of their residents.
I still see this person hanging around the street daily, tagging would certainly help the charity keep a check on the ones who may need it.
Oh course this is not NHS run so I don't expect anything to happen soon.