Hi, I am on ONO and I got a little further: if you download files by searching (try a small audio file from the Golden Bough, or something else classical), you will see them being saved in .../persistent/librifox/app_dl/1113/ file (on my system at least), in .lfa format. Sadly, converting existing mp3s to that format in the folder does not produce desired results. Will keep you posted.
Just FYI someone did manage this successfully on ONO before:
http://weblog.elkner.net/2018_06_16/