I think I figured out how to bridge something like SSB into the IndieWeb. In short, a n-count hub, one for each user, that handles the work of following and subscribing to people in the space as well as handling messages from the IndieWeb into the SSB space.
I have ideas on how this might work. I need to sit down and finish what I have built for the fusion of Lwa and Lighthouse so I can make this simple. The goal, by the way, with the new Lwa is to take all of the logic for Webmentions and fold it into that service. That way, people can get a in-built reader for their mentions. I might aim to handle authorization of feeds as well. Perhaps also a button to handle registration with things like Twitter and Mastodon via Bridgy.