Many many many years later…
I would like to close the loop here, especially since @pekka made a welcome surprise visit on meta today 
There are 2 things we have done to help in this department.
Firstly, we release the official Discourse Reactions plugin. I find it incredibly valuable especially in closed communities. I do wonder if it makes sense to try it out on meta or not.
Secondly, we have a literal … “Sam has read this” feature that can be enabled on group PMs.
This was first brought up by @jomaxro (we got to close the loop there) Ability to view if PM has been read (Moderation ability?) (and also mentioned by @tobiaseigen )
Announced here: