Maintenance Helper Module

Every time I migrate a site (which seems to happen about once a month on average...) I find myself in a bit of a debate: I don't want to put the site into maintenance mode because I want people to be able to view the site, but I do want to prevent users from creating new content that would get lost in the migration. Ideally you can cut this period down to a half hour or so through the use of short DNS "time to live" values, but there is also some amount of time where you have this problem.

I'm thinking of creating a helper module for this situation that would provide the following features:

1. A new permission for "Log in to site" - since permissions require a valid user object that is only populated after the log in this would be evaluated just after a user logs in. If they don't have the permission, I would set a message (admin configurable) and redirect them to the "/logout" page.

2. A configurable message to show to users that would say something like "Hey, we're doing some maintenance now and have therefore restricted the ability to login to the site. Sorry!" This would warn users before they attempted to login so they know that they are probably wasting their time if they do it.

3. A button to clear out existing sessions that users have for roles which do not have the permission to login. The protection in item 1 is pretty useless if you don't also get rid of existing sessions.

So, does something like this exist?

If not, what to call it?

"maintenance_helper" seems a little long, though it is descriptive of the purpose. "login_permission" is more accurate but doesn't describe why you would want it (which is both good and bad, I guess).