I'd say "don't do that" because that will lead to a whole lot of problems: most svn clients are not prepared for that, and the automatic tests of the svn library don't test for that, so if commands work with such a repository it's pure luck.
Another thing:
You can also create a backups on client machines if you like. First create an empty repository, then enable the pre-revprop-change hook.
$ svnadmin create backuprepo
$ echo exit 0; > backuprepo/hooks/pre-revprop-change.bat
now you can use svnsync to synchronize your repo on the server with the local one:
now, all you have to do is run
$ svnsync sync file:///d:/backuprepo
every day and you'll have a mirror of the repository on the server.
I do that with all my opensource repos as well - sourceforge lost once my repo and their backup was corrupt as well, so now I keep a mirror of my repos just to be save.
Stefan