If you want to follow the same pattern as the plugin, and simply modify the reply permissions, then it’s relatively simple. Maybe 1-2 days work for someone familiar with Discourse programming. I can see how this would be useful for a Q&A style site.
As Jeff said, trying to modify view permissions per-topic is very difficult, and is unlikely to ever happen. That’s because it would require extensive changes across the whole application including topic lists, suggested topics, notifications, etc. If you need to modify view permissions on a per-topic basis, then use PMs.