In the Platform Explorer, I see Extension point serviceDefinition of component CoreManagementComponent allows a message to be set by an Administrator that appears on all pages of the Web Application. This is a useful feature that does not appear to be exposed on the Admin Console nor does it appear to be documented anywhere except deep in the Platform Explorer.
If I want to set the message in real-time in the current release, must I resort to hot-deploying a bundle with the appropriate extension point specified? Exposing this capability in the Admin Center would be much more convenient. Even better would be the ability to set this message globally across all Nuxeo instances in a clustered environment.
It is exposed in the Admin Center: chose tab "Monitoring", and click on the "Edit status" button for the "Administrator message" item. Then you can set a message that will be displayed as long as you keep this status set to "Active".
The extension point makes it possible to add more custom ones, registration of these statuses is quite simple (just take example on default ones: https://github.com/nuxeo/nuxeo-core/blob/release-5.5/nuxeo-core-management/src/main/resources/OSGI-INF/core-management-contribs.xml)
Then you can write custom code that will check for this status (or listens to this status changes) to do things like displaying a message, disabling a task usually triggered using a scheduler, etc...
I think these are the main links to see how all this works in the case of the admin message: