I like the feature idea! It's unlikely to make it into core anytime soon unless a community member submits a PR (leading up to stable and immediately beyond, our focus is fixing bugs and enabling extensions to do more), but it could be a really nice tweak to the design!
I'm not sure about the particular UX proposed above though: I feel like having the indicator on the left isn't too clear as to its purpose, it's too far from the rest of read-post-information. Perhaps the unread posts icon on the right could somehow be changed or use a different icon? Maybe bold and black text, but not filled in comment bubble?
