I had a look and the only thing of use I could find was open source code. So I dropped a line to Thorsten Lemke, author of GraphicConverter, because X-Face is one of formats GC can encode and decode, saying "The only thing I can find is source code at http://freshmeat.sourceforge.net/projects/compface/, and it may be that the only information on how to decode and display an X-Face is buried in this code (which I confess means nothing to me)?"
His reply was "yes, just use that code as base for understanding the encoding".
So, in essence, I think it means that unless you can understand and use/translate/transcribe that code, we've had it.
But you probably knew that already ☹️