This is an interesting plugin idea which I've seen before on other software.
On this hook you want to save into database list of users who have replied.
Likely this hook: https://github.com/NodeBB/NodeBB/blob/master/src/posts.js#L99
If user has not replied to this post (also check if this is the first post in the topic) then replace content with "Please reply to this post to view content"
Something like that. I spent all of 30 seconds thinking of this maybe it's not 100% right, but good luck and hope this helps