Which mBaaS support SiwA?

I like what SiwA could mean for my apps, and now that I'm re-writing my mac app in UIKit so I can deploy on both iPad OS and macOS, I'd like my auto-renewing subscription users to only subscribe once, not on both platforms. But I don't have time to build a lengthy server infrastructure just to support syncing the subscriptions with a database. Which mBaaS companies support SiwA? I already checked backendless, Kinvey, Kumulos, Firebase, and sashido, and didn't see any "drop your SiwA certs here" instructions, and none of these companies has responded to my requests from their pop-up chat bubbles.