Anytime an update comes out for an add-on or style, the installation freezes the forums. I would like to be able to send an alert to all online members with a notification "the site is undergoing maintenance and may not be responsive for a few minutes". I am not aware of any current way to let users know about the maintenance other then making a forum post which many wont see.
I do have this add-on installed but to the best of my knowledge it allows me to send alerts to user groups. I would prefer not to send an alert to all registered users when it only pertains to those who are online.
I would guess a modification would need to be made to create a user group which adds/removes users as they go online/offline?
I think this particular piece should be in the core. Anytime a site needs to perform an update you will have every user start to post/msn/etc the site is down when really it is undergoing a planned maintenance. For my site, I will set a planned maintenance window, post a thread, etc but there will be users who miss the notice.