A possibly slightly better approach might be some sort of automatic-login plugin that creates a dummy account for a user and logs them into it - that way the user has an account to submit from. I don't know if there is such a plugin in the Moodle plugins directory, but, from what I remember, it isn't that hard for a Moodle developer to create something like that.
However, I'd be curious as to what the point is of an unregistered user submitting an assignment - if the submission isn't linked to any user account, how can it be assessed and the feedback returned to the correct person?
However, I'd be curious as to what the point is of an unregistered user submitting an assignment - if the submission isn't linked to any user account, how can it be assessed and the feedback returned to the correct person?