Cool, I could see where stats collection to that extent would prove useful. But it would still consume storage & resources, and you're going to run out of said resources/storage unless you've got a huge datacenter (like FB, FBI, NSA... here's where the HA HA came in with my first post)
Hmmm. For the purpose/use you have expressed, implementing an "On/Off" option (in thread options) would make sense, but not as an after thought.
It would make more sense to run 24/7
Dumping every 24 to 48 / 72 hours or etc (depends on traffic / content).
The On/Off per thread options would simply be the option for moderators to trigger that thread data be retained during the next scheduled dump. (of the data the addon would temporarily store)
Eventually you'd just get tired of apologizing for other people, and probably find that your regular members are the ones that you have to do the most apologizing for. Wolf pack mentality and all that fun stuff.
It would actually use little to no additional resources or storage. Most of the data is already stored. Think about how XenForo tracks what messages are read/unread and whether there are new posts in a thread. It already stores enough information to know who has or hasn't viewed a thread.
On my forum this was a very exceptional circumstance, so if the addon was resource hungry I would weigh that against how often we'd find it useful and probably not bother.
In the end, instead of a few discrete PCs to those who saw the thread, we did a blanket announcement that was not too specific, ie "we had some unpleasantness recently..." and as it has happened this has worked really well. Those who saw the unpleasantness were happy we noticed and bothered to take the time to explain, and we got loads of members who just reaffirmed what a lovely forum it was.
So if I never posted this, left the thread, came back, and then left again without posting, and then came back in ten minutes, posted a smiley face and then left... there would be data in the DB that you could extract (if you had DB access) and tell exactly how many times I entered this thread without posting, and what times I entered?