I wrote some code for the BC Libraries Cooperative for issuing a maintenance message to JSPAC via an apache environment variable. Thus, you merely add a line like this:
SetEnv MAINTENANCE_MESSAGE "We will be down tomorrow night. Please use the card catalog at that time."
Then apache-reload and a red banner will appear at the top of the OPAC with this message for patrons.
Porting this to TPAC was not difficult, so I have done so and finally am sharing the code here now for your amusement.
Not sure if SetEnv is the best way of doing things in a post-JSPAC world, but it works quite well for notifying everyone who uses the OPAC, particularly any cranky nightowls who may want to use the catalogue while you're doing your maintenance after hours.
user/jamesrf/maintenance_message
Note: I had to add the code to two places in the JSPAC because the home screen (home/index_body.xml) and the rest of the JSPAC (body.xml) have different locations for their <body> tags. For the latter, body.xml, it was necessary to wrap the main table in another div tag, otherwise the locale selector winds up within the alert area and tends to block particularly long messages.
Fancy, I was just thinking about writing something like this for our libraries too after our last couple upgrades made me feel like we weren't giving folks enough warning.
Not entirely sure about making it an apache variable and not something in config.tt2 or similar instead. Going to ponder that a bit and see if anybody else has opinions about the feature.