We’re currently building OAuth2 support for Gitlab (open source, on-prem Github alternative)–so anyone with a Gitlab account would be able to use those credentials to sign-up for Mattermost and create an account.
That functionality is aimed to ship end of this month with our v0.6.0-preview release, and that framework could be adapted in future to do what you’re looking for. Maybe take a look in a couple of weeks, and maybe it narrows down the additional work that’s needed to integrate your app?
That said, we don’t have tentative timelines for general purpose OAuth2 and API documentation (which depends on which APIs we feel are sufficiently stable to document). The mobile app we’ve been using internally, but we put it on pause to get a bunch of other items in for v0.6.0-preview and it’s an item in the backlog to sequence for future sprints.
The best way to influence the roadmap and timeline is via the feature request forum: http://www.mattermost.org/feature-requests/