I'm not quite sure what you want - do you want to backup every version of every file sent to the server, or have a copy of the latest file saved somewhere?
Either way, you're not going to have much luck - the client only sends the changes (the delta) to the server, so even if you set a server-side hook that wrote the revision data to disk, it'd only write the changes.
What you could do is start a new client, in the post-commit hook, you could trigger the client to get the latest revision that's stored in the server (you'd make the client do svn update on an existing checkout, or svn export to grab a read-only copy).
Generally though, you do not need to do this. SVN is very safe, so you really only need to look at backing up the repo. My favoured way of doing this is to run svnsync so you always have an exact copy.