Custom user authorization in authz docker plugins?

From documentation on docker authorization plugins:

If TLS is enabled in the Docker daemon, the default user authorization flow extracts the user details from the certificate subject name. That is, the User field is set to the client certificate subject common name, and the AuthenticationMethod field is set to TLS.
(end of quote)
Is there an alternative to TLS in plugin API, such as the way to implement user authorization based on system user identity?
Thank you,