Best way to perform application level high availability authentication

Im creating an app thats using CI for the back end. This question isnt specifically for CI, more for applications in general, but I figured this might be a good place to ask.

The app stores the accounts in a MySQL database that it uses for authentication. You can login to the app and submit support tickets, view logs, etc etc.

What I wanted to do, is allow people who have have the application installed, to be able to still login to their app and submit a ticket, even if the MySQL database isnt reachable (Say an update went bad, or a plugin broke something and the app can no longer connect to the database...).

What I was thinking, is in the PHP, whenever accounts are created/updated/deleted, just mimic the actions to a SQLite file. Then when the MySQL isnt reachable, fail over to the SQLite file for authentication. and in the PHP, just do some logic to see if the auth was done to the SQLite file, if so, only allow certain features.

What do you guys think about this idea? If you dont like it, is there a better idea?

Since im only providing the application, I have no way of making sure MySQL is HA or even backed up, but id like the customers to have the ability to atleast login and perform some basic actions, for "emergencies".