Did it finally. List of steps (with parameters for MY situation, be careful if you copy this, you are root!):
(0) transform /dev/sda12 from reiserfs to ext4 (obviously not if you already have ext4):
# fstransform /dev/sda12 ext4
The tool is SEMIautomatic, so pay attention! It instructs you how to monitor it so you can kill it with Ctrl-C before it damages data. Basically the file system needs to have at least 10% space free to be transformed safely.
(1) make the installation SD stick and the bootable SD stick following instructions, check that the latter works.
(2) format /dev/sda1 (the 300MB so-far-useless partition) and mount it:
# mke2fs /dev/sda1
# mkdir mountpoint
# mount /dev/sda1 ./mountpoint
(3) copy the KERNEL and SYSTEM files from the bootable SD stick and create their checksum files (I did that even though there were no checksums on the SD stick):
# cp /media/System/KERNEL /media/System/SYSTEM ./mountpoint
# cd ./mountpoint
# md5sum KERNEL > KERNEL.md5
# md5sum SYSTEM > SYSTEM.md5
(4) edit the file /etc/grub.d/40_custom adding the entry
menuentry "LibreELEC" {
set root=(hd0,1)
linux /KERNEL boot=/dev/sda1 disk=/dev/sda12 quiet
}
and update grub:
# update-grub
When I rebooted and selected LibreELEC from the grub menu, everything JUST WORKED. The new directories created by LibreELEC on /dev/sda12 peacefully coexist with my Debian installation which also mounts /dev/sda12. (I first looked at the live stick to make sure there would be no name collisions.)
Thank everyone for their friendly input (I even ended up making the stick, and it did not spoil the party).
Denis
PS The laptop is a first-generation HP EliteBook 2540p with Intel on-board graphics. Originally HP had configured that 300 MB partition with a quick-boot Linux which launched into a browser directly, but it never worked for me because I trashed the MBR when installing Linux. So basically it is their idea.