With regards to the privacy concerns of Research mode, there may be a way solution. For sites like Reddit, it should be possible to build a bloom filter. Have the metafruit server actively spidering Reddit for new, popular threads and add them to a bloom filter. The plugin would download the bloom filter from the metafruit server at some regular interval. That way checking whether any particular URL has an associated conversation is just a local operation. Plus, it's faster than pinging an API, and burns less of the target API's resources.
That would also provide a way to monetize, by giving out the metafruit bloom filter to subscribers only. Or perhaps the free plugin can update its bloom filter once a day, but subscribers can update once per hour.