An XDA contributor has observed that it is now possible to control media on their Chrome OS device even though the screen is locked. The feature has not been part of this system by default in the past, but is reportedly there in its 79 Canary (or 'advance') version. It has been shown working for YouTube content.
https://www.notebookcheck.net/Chrome-OS-may-get-lockscreen-media-controls-soon.435134.0.html