So you want unique accounts, but dont need to know who used the account?
Like the standard anonymous self-registration, but not quite?
I'll tell you what I did and you'll tell me if that solves your issue.
The customer requirement was "Accept Terms" only, but still give a unique account to each user.
Using the Allow anonymous weblogin does this partly, but use the same account for each user.
It's been a while so I dont remember the exact details, but to work around this I basically created a normal self-registration page with accept terms checkbox as the only visible field, made the username field hidden and enabled NAS login. That caused an auto-generation of username/password with direct login option on receipt page..
If you cant get it working let me know and I'll look into the old code and see if there was any special tricks to it.