I was planning to use user_touched and/or user_editcount on server side to determine a cohort and then pass that via the makeGlobalVariablesScript hook to JS.

That could be inconvenient since we cannot analyze past data, but have to wait for new data to be collected every time we define a now cohort. In my experience with MediaViewer, though, almost all our data-driven decisions were based on data which we collected with a specific purpose in mind. We collected lots of data with an "it will probably be good for something" mindset, and it turned out to be not so useful - whenever we wanted to use it to answer some specific question, it turned out that there were some small mistakes or inconsistencies which made it questionable, and which we would have surely catched had we set up the data collection with that specific question in mind. So I am not too worried about that.