Bumping this topic!
This would be a super useful feature for us who use NodeBB forum for storytelling purposes. In our forum, each topic is almost like a diary, and each post of those topics can be about totally different subjects. Tags to posts would make following different story lines so much easier for our readers.
We recently moved from phpbb3 to NodeBB, and post-based tags is literally the only feature I miss about our old forum. I really hope this feature will be added to NodeBB.