I was wondering if it's possible to add a local file based failsafe. A backup really.

So that if my MySQL database is down it won't affect gameplay.

Inconsistent state is bad.
In my opinion the best solution is to deny all logins until the database comes back online.

I don't like the idea of a "fallback" - it shouldn't be necessary if you have your database setup correctly.

plus: other plugins are likely to experience issues if the DB is down anyways.

Going to deny for that reason - thanks for sharing the idea though.