This will work, but only when the user does not already have a private page defined. Once the page is created (i.e. their account is created), this will no longer apply.
You can also use the site templates feature to define a template which would apply to the pages. In 6.0.x and earlier, same issue as above applies (template only applies to new accounts). In 6.1 and on, dynamic templates allow for a template change that will apply to all accounts.
