Generic pubsub over Matrix - room participant privacy?

I was thinking about using Matrix as a generic pubsub mechanism, and I realized that the only required change would be the ability for rooms to not publish their participants in the room state. After doing so, users could "subscribe" to such a room by simply joining it.

So for example, users of an RSS-like application over Matrix could join a room for each blog, and their reader would receive updates as new posts are made and published to any of those Matrix rooms.

I haven't yet thought about what this would actually look like in the spec in much detail, but what do you guys think?

Start by reviewing the Matrix room state model and how room participants are currently published. Compare that behavior with the proposed generic pubsub and RSS-like use case in issue #249. Done would require a concrete specification design for private participant visibility, including how users subscribe by joining and how the behavior is defined.

Written by the indexing model from the issue text.