You know, here’s a good idea for a plugin that would solve this given that you don’t use custom URIs:
A plugin that is enabled can enable the administrator or other users with a given permission level to go to a certain Option-page. From here they get a check-box and a textarea where they can write a message to the users who want to get in.
When the checkbox is marked, a file called index.html will be created in the main directory. .html files will normally be loaded before .php files by default. This “placeholder”-file will include the message that the administrator wrote, and a link to the Site Administration page if the administrator allows this. Adding wp-admin/ to the URI will get you there anyway, but it makes it a little easier for the sub-administrators.
When the administrator then unchecks the “Take website offline” checkbox, the index.html is deleted and wolla – website is back online again.
The placeholder-file does not need to be linked to any templates at all if you include all the HTML in the plugin.
Any thoughts to this approach?