Actually, I once wrote a solution you may be able to use.
It's an action-plugin that sends notification mail to a certain group of users when a page is added to a namespace (I use it for that, but you could easily modify this to be an admin-plugin with post-form). My suggestion is that you could use one namespace for maintenance-messages, and have all users of a certain group being notified when you add a message to it.
Mind you, I have not tested this more then I found necessary - it works well to send out copies to people who normally do not read the wiki. My page is more of a complete news system.
It does not allow one to subscribe to all subnamespaces (e.g. news:*), only pages added in e.g. news. I'm planning on adding this with regex support at some later stage.
You have to check the "send notification mail" checkbox in the edit form (which only appears in the right namespaces - if it doesn't appear, but haven't setup the namespaces correctly) to actually send the mail. I'm guessing you could use group "" (empty) to send to all users of the wiki - but I have not tested this.
You can get it, (and use it at your own risk) from:
http://folk.uio.no/twhoffma/dokuwiki/nssubscribe.tar.gz.
It works well on several namespaces, comma-separated - but as far as I know - not on several groups.
For your second problem, I would simply setup a rediect to a static html-page while you update the wiki. You definetly don't want any of the scripts on the wiki running when you upgrade. Say maintenance.html in your webroot, and redirect from your index.php file.
Good luck.
//Falkor