https://engineguard.com.auI have fitted quite a few of these. They bolt on as you say, have programable alarms and can also have a low oil pressure alarm. They can also tell you the max temperature it has reached, this is nice as I normally set the alarm to 95c and then after the machine has been run I can turn that down to say +5c on the operating temp so any issue is caught sooner. I've mostly fitted them to tractors for farmers, tractors tend to have so much cooling system and fan they rarely get hotter than thermostat (normally 82c) unless the radiator gets packed up with stuff.
As it's an alarm system I've hidden a few up under the dash so people cant stuff with them. One I hooked up to a car horn, you blimmin well crap yourself every time you turn it on (on boot up it toots so you know it's working) but the farmer was really not keen on another engine rebuild after the failed water pump...
At least in my experience the temperature gauge will still read if you lose all water as they conduct heat from the metal they're threaded into.
In my opinion an over heat alarm is totally worth having. You don't tend to look at the gauge unless you're having problems and overheating absolutely kills these engines.