Well since its a fallback mechanism it should only come to action when no connection can be made
with the MySQL databaseserver so as for overhead I really don't think its a problem.
I meant with regards to the admin side, when you update a page, settings, user etc... the change would have to connect to both databases and update them both to keep them up to date.
Unless, a cron was used to back up to the backup database every night, then it's best of both worlds nearly. The backup may not be 100% up to date but would be up to date enough to run the site.
The cache method BlueWolf mentioned, again in theory would work but by my understanding of it, considerable changes would need to be made in order to be able to revert to these cached files AND allow you to still be able to login / update and manage the site as thought it running of a database. You've then also got problems of, changes made into the cached version, what happens when the database comes back online, there would have to be a way to sync them.
His last suggestion was probably the best on so far, just get another mysql server and keep it up to date.
Manually changing the connection details would also probably be the quickest way.
At the end of the day, all of these are (probably) implementable (if that's a word) it's just whether it would be feasible for it to be put into Wolf and whether or not it would clash with the project goals. Would be up to Martijn, if he feels it's a feature worth adding / whether it would be feasible. My suggestion would be to add it to the issue list as a possible feature, and take it from there.