More of a documentation area now, just in case there's anyone else out there that still uses this ancient player.
The reason behind my original ask was that the player I originally had was a "Kong" version of the X20. The Gigabeat start / shutdown logo was of King Kong and the firmware showed up as a 1.001 JP version. I wanted to find a way to eradicate those logos.
Recently two things changed for me. First, I ended up buying an X30 with the standard firmware (1.000 JP), so the Kong20 is now a backup device. Secondly, I learned about an old development tool here called "Gigabeat Flashwriter", probably last developed in 2012. This was tested mainly on the F, but I managed to get this to compile and work on my X to make backups of both my devices.
And I did get it to actually work to erase and flash my Kong20, using the X30 backup bin file.
Not for the faint of heart as honestly I figured I had a fair to moderate chance of bricking this thing, but amazingly I did not. I did not attempt the fast RB bootloader though, not sure if I want to chance fate.