Couldn't the same thing be done by a wristband with just a shortened weblink. The advantage is you could update the website and the link could be engraved on the wristband.
I'd have thought a wristband would be too easy to damage/remove - especially if they become indicators of people in that kind of situation. Similarly I think things like tattoos or embedded chips are spectacularly bad ideas - for all manner of reasons.
You could make the wristband hard to accidentally remove, but if it is made out of a decent metal and engraved then it would not be hard to ensure that it survived.
Something like this needs to be voluntary - we really don’t want to be forcing vulnerable people to start wearing something they don’t want to.
Maybe retina scans or something like that?