Got an email from someone asking about this - namely, using a third database/table/field that contains information about permissions for the accounts in question.
In theory, this is a non-trivial addition, except for the complete lack of standardization of how to actually determine the permission levels. Right all user accounts are created with the same level of permissions as set in WP config section.
Is this a feature that others would use? Is it necessary? Based on feedback, I'll work on adding it or not, as I personally don't use this.