Following suggestions by Pali, I am looking at rescue OS:

https://talk.maemo.org/showthread.php?t=75369

The files are at:

https://n900.quitesimple.org/rescueOS/rescueOS-1.3/

What I have done up to now:
- downloaded the kernel zImage
- used mkimage to convert it to uImage, with mounting address 86008000 and starting address the same
- with reference to Kicherer's blog, made a script for loading the uImage to memory and booting it. Simply
setenv bootcmd 'fatload mmc 0:1 86008000 uImage; bootm 86008000'
- converted the script into image boot.scr, address 0
- placed uImage and boot.scr in sd card (single partition)
- if I insert the sd card and boot from there, rescue OS starts

The problem is that it doesn't see any file system (I think); it gives an error saying the root is not valid and goes to kernel panic.

Now I contacted again Pali and also NIN101. What I have understood:
- the remainder of the OS is in the file rescueOS-1.3.img, same link as above for the kernel
- this file is to be converted to uImage with mkimage. Did this with name uRam:
mkimage -A arm -O linux -T ramdisk -C none -a 82008000 -e 82008000 -n ramdisk -d rescueOS-1.3.img uRam
(the address 82008000 is a guess. I think I understand that addresses should be multiples of some quantity, otherwise the kernel says we are unaligned)
- copied uRam to sd card
- loaded uRam to memory by fatload (address 82008000)
- added to boot.scr the line:
setenv bootargs 'initrd=82008000,8M'

This does not work. Clearly I did not expect it to, since I am just trying and trying, understanding step by step as I make errors.

I have posted all this in the hope that somebody may correct my errors and suggest what I should do now.