The 'any account' option is only a very partial solution, but in some way it could substitute the 'all existing accounts', so you have not to add nothing in those folders where you want access for all registered users.
But here is a (not very clean) workaround for dardarlt's problem:
Create a folder where only anonimous users have access.
put in that folder some text files, each with a user-password combination, in such a way you have previously generated the users by hand
Make a difftemplate for that folder that will display only the first file that was never downloaded.
Put in your main page a direct link (named something like: get account and password here) to that folder, make them download the unique visible file. Once they login, they won't see anymore that link as they are registered users.
If in that way you create perhaps ten or twenty users, you could make the passwords expire, creating separate logfiles for the users. When you see that a user does not connect in some days, simply change password in the file and in useraccount.
If that may be useful for you, try it. If not, forget it.