I'm looking for this same feature. I'd love to provide a Flarum community to my patreon supporters who are supporting me at certain levels using SSO through Patreon's API.
I could probably come up with a budget for this and would be happy to donate the plugin to the Flarum community. Is anyone interested? What kind of budget would be needed?
Also, is Flarum stable enough to use with a small group of people? I'd hate for people who are paying for access to my Flarum community to bump into issues with it. (For example, I still can't confirm my email address here. I get the error, "Invalid confirmation token," whenever I click the link in my email. And I get an error whenever I try to upload a profile image.)