is there a way to find out, if there are any users
currently logged in to a wiki?
I'd like to check this, before closing down the wiki for maintenance/updates, so
I don't involutarily kill the work of a user before he or she has time to save it...
Not really no. Being logged in just involves having a cookie on your
hard drive - there is no way for the server to know who has such a
cookie except when they are actually making a request.
You could probably do something to tell if someone has clicked "edit
this page" in the last 10 minutes and not saved yet, but that's the
best you could do (and I don't know of any extension that does that -
you would probably have to write it yourself).