by Daniel Neis Araujo.
Hello,
as far as I know, the standard "webservices authentication" of Moodle is used to authenticate consumers of Moodle's webservices.
If you are interested in authenticating users against an external webservice, you may take a look at
https://github.com/danielneis/moodle-auth_ws/
It works with SOAP, but feel free to make it work with REST and other protocols and submit a pull request =)
Kind regards,
Daniel