Here's how I would try to decode the bytes: Make a MIDI on your Yamaha machine by doing a step recording using the CHD tab. Save a copy both before and after you "expand" it. At least one of them will have the XF chord events present. Make a note of what chords you entered, and analyze the data to find a pattern.

It's been a very long time since I experimented like this. I think what I was doing was using a spreadsheet tool to manually create and edit the meta events. Then "concatenating" (appending) multiple cells together and pasting text onto the end of the MIDI file. Definitely possible, but awkward.

As I wrote in my original post 15 years ago, building a tool that does this shouldn't be too hard for a skilled GUI programmer (which I am not!) Good luck!