I was finally able to decompile some of the lua, and now I see where some other people got their code.
Anyhow, I'm not sure what to do with ZO_Stats.control. Any references to ZO_Stats.control, ZO_Stats.controls, ZO_Stats.Control, or ZO_Stats.Controls all say "nil", and iterating through all the k/v pairs in ZO_Stats shows nothing but functions. I also tried ZO_Stats["control"] and got "nil". Is there some other way to see the control variable besides d(ZO_Stats.control) or trying to iterate through its k/v pairs?
I tried using the ZO_PreHook function to get the control variable passed into ZO_Stats:Initialize, but I can't figure out how to make the prehook work. The best I can do is not get errors popping up -- the prehook itself doesn't seem to do anything.
I was able to get something passable by setting my custom control's parent to the stats window (so it shows/hides with the stats window), and formatting the text so it looks like the normal stats, but I had to stick it by the Active Mount title so it showed up, and it's fairly narrow to fit in that space. I'd rather just bump the other stats down one and insert my control at the end of the StatRow items.