I tend to agree with @ErmesT the system to do this is already in place through the right channels. While I like the idea from a safety perspective, I also understand that the economic impact of marking a major city say New York, London or even little old Melbourne where I live as some disaster happening would actually be more significant than any terrorist action or other disaster. It is a challenging situation and would need to be considered carefully.
Australian tourists (and anyone else who felt like using it) can subscribe to a quick notification system for when they are travelling https://smartraveller.gov.au/Pages/default.aspx
Regards Paul