Just wanted to stop by and mention that this is all looking very good.
As one note, the use case for "virtual" streams seems somewhat unclear to me based on this document, maybe an example could be added?
Topic on Talk:Requests for comment/Multi-Content Revisions
Good point, I'll think about how to make it clearer. The idea of "virtual" slots is that derived content doesn't necessarily have to be generated when the primary content is saved, and stored along with that content. It can also be generated on demand - which would be a virtual slot. The any calling code, this would be completely transparent. One possible use of this is getting the HTML rendering of a page. Even though it seems obvious enough, it's not a good example, since it raises a lot of other issues (different renderings for different target platforms, updating when a template changes, etc).