As of 4/22/2020, still nothing to update the timeline_comments except a slight possibility of posting to:
which I never figured out.
You can inspect an order page and examine the POST to the "Timeline" using the "Leave a comment" textbox (F12 in chrome). I had a work-around so I never tried all the header and cookie combinations it might take to use that endpoint. It looks like you need a valid login or session to use the interface.