Blake C:Something that will allow login, but direct users to a screen with a message about the temporary down time?
In short, no but what you could do is temporarily disable their login from the Customize > Users screen under Edit for the user you want to disable.
Click the Login is Active checkbox and the next time they load a page they'll be booted out. This can have some undesired effects if the user was mid-way through processing something. It would not be recommended but if there was an emergency and you needed all users off the system, this trick would work.
If you update a page while a user is on it they will get a message that the page have been republished and it'll need to be reloaded. It sounds like to want to completely take them off the system so I think the best thing might be is a company wide email asked users to logout, then make their logins inactive while you make the changes you need.
Alternatively, you can work on your changes in a separate account and use our copy/import screen feature to import the changes.
~C