Hi Daslicht – the code in that panel is either too clever for my understanding and on another level, or way off mark – I’m really not sure, but it’s probably a waste of time looking at it. My panel is known to work for the JD-990, but I’m not familiar with the JD-800 (ie don’t have one). I tried to PM you; didn’t work, but you can find my email address listed on the panel if you go to panel author email.
This post lists the differences, but I think the sysex is all different if I remember correctly – It might be possible to just change over the sysex to start with and then add and (mostly remove?) certain features of the panel
https://www.gearslutz.com/board/electronic-music-instruments-and-electronic-music-production/239173-roland-jd-800-jd-990-a.html
Cheers,