I have many ideas for how this could work and we need to bash them around to find the simplest.

The underlying structures we have on Hive are such that sharing text and information is trivial.

What is missing is some agreed way for front ends to recognise when such a signal has been sent and display it to all users in some way.

First suggestion from me: a standard post format and then when enough top 20 witnesses have signed it with a common comment response, like "Co-Signed by Witness Name" front ends can make sure it is alerted in their front end in whatever way they deem appropriate.