For what it's worth, I've tried experimenting with this on test.d but I haven't had any luck thinking thru what exactly would be needed or how it would be implemented. If someone has feedback and wants to collaborate, please let me know.
This is a tracking task so probably many more subtasks should be created (I honestly haven't even looked at what would be involved in doing this). I created the task as a starting point because I couldn't find a Phabricator task for the proposal. I started looking into this because of T187900.
I'm sorting out the hackathon 2018 board (on which this task is) and well described tasks generally have a higher chance of actually being worked on. Might not be the best board for a tracker task.