fwiw, there is a new Discourse plugin on the scene. https://phabricator.wikimedia.org/T124691#3874322
It is create by my intern. We could also investigate locking down the profile email field so it cant be modified from the email address that was obtained from Phabricator. We have already investigated adding Discource user profile flags to indicate roles a person has, based on information that can be obtained via Phabricator, such as repo 'committer', project membership, etc. Looks very feasible, but needs a bit of work to implement.
A Phabricator based SSO and profile may prevent some people from joining, but it will also allow many, many more to join with a few clicks.