A common mistake would be the forgotten semi-colon and/or _prepend:

FILESEXTRAPATHS_prepend := "${THISDIR}/files"

Another common mistake is to not respect the tree layout of the original
path relative to the original recipe (or for that matter, the one you
want to override coming from the bbappend) for xorg.conf.
In that case, you can see that in meta-yocto-bsp, there's a parent
directory of xorg.conf that is not the one listed in
FILESEXTRAPATHS_prepend. You should add this directory in the path of
your bbappend.

Indeed, I was basing my layout on the original recipe, not on the poky override (which adds the $MACHINE as a subdirectory).

With that adjustment everything appears to be working fine.

https://pretalx.com/media/yocto-project-summit-2021/submissions/WTT3UV/resources/Demystifying_the_OVERRIDES_mechan_no6J6fb.pdf
slides 70 to 80 might help you. The recording should be available in a
few days/weeks.

Thanks a lot, this looks really useful ! 

-- 

      ...ich hab' noch einen Koffer in Berlin...