Well, I've done without mtdram.ko using the instructions above:
https://www.digitalworldz.co.uk/index.php?threads/160169/#post1168973
Instead of creating /dev/mtdblock/7 I have used existing /dev/mtdblock/1.
So, all steps shortly:
0. I have made changes in /var/tuxbox/config and my Dreambox 500 was not able to boot.
1. Created backup using DreamUp.
back.img
2. Found jffs start position (it was 500000), decimal 5242880.
3. Extracted "bad" jffs
dd if=back.img of=split-jffs2-section.img bs=1 skip=5242880
4. Restored clear (working) Gemini image to dream.
5. Copied split-jffs2-section.img to /tmp of my tuner.
6. Mounted instead of current jffs:
umount /var
dd if=/tmp/split-jffs2-section.img of=/dev/mtdblock/1
mount -t jffs2 /dev/mtdblock/1 /var
7. Rolled back my changes in config
8. Got fixed jffs image
umount /var
dd if=/dev/mtd/1 of=/tmp/changed-jffs2.img
9. Copied it back to my "big" pc and got new image
dd if=changed-jffs2.img of=change-full.img bs=1 seek=5242880
10. Flashed dreambox using DreamUp from change-full.img
And finally I've got it working. Many thanks beady for information.
If I will have to repeat all of this, it will take about half an hour.