As I said on wikitech-l, we should improve the message shown for $wgReadOnly mode when switching datacenters. The overall message is defined on translatewiki and takes an explanation as a parameter (See the manual for futher information). What we currently insert as this parameter there is
"MediaWiki is in read-only mode for maintenance. Please try again in a few minutes."
We should improve this message to clearly state that we are about to switch over datacenters and link to the meta page: https://meta.wikimedia.org/wiki/Tech/Server_switch_2017
When switching DCs, it is still necessary to edit the configuration files in rOMWC by hand, upload them to gerrit, merge and git pull them to the deployment host: Example. So it would be fairly easy instead of just commenting that lines or uncommenting them to just choose a message that is actually specific to the switchover we're doing at the very moment. The problem is that we currently have the generic message hardcoded in the switchdc scripts t02_start_mediawiki_readonly.py and t08_stop_mediawiki_readonly.py, so it seems in reality it isn't as easy to exchange that message as it should be.
BTW I couldn't find the operations-switchdc repository in diffusion (gerrit links to the diffusion repo don't work either) but only on github.