It should be possible to set JAS in maintenance mode. This should show an "under maintenance" page for all projects.

Since this the goal is to allow for upgrades of various systems (include JAS itself) it should limit dependencies as much as possible. I.e. the earlier page can be shown the better, keeping in mind that JAS itself should remain accessible. 

It would make sense to have a this in a config option in the JAS project with a configurable text. Once enabled the text should be shown on all the various GUIs and APIs (Data, XML, CSV, etc).