Thank you so much for taking the time to respond. It’s very much appreciated.
Following your instructions mentioned here, I took a look at the inside of the container again.
root@core-deconz:/# mount
[...]
/dev/mmcblk0p8 on /data type ext4 (rw,relatime,commit=30)
[...]
This leads me to expect that anything stored under /data will survive a container upgrade.
So, I established a new fyrtur[…].json file in that partition.
root@core-deconz:~/.local/share/dresden-elektronik/deCONZ/devices# mcedit fyrtur_block-out_roller_blind.json
I pasted the contents of this source into that json file:
https://raw.githubusercontent.com/dresden-elektronik/deconz-rest-plugin/v2.23.x_stable/devices/ikea/fyrtur_block-out_roller_blind.json
I noticed the same file in the v2.24.2_stable and master branches both list "status": "Draft". 
That is why I used the older version from the v2.23.x_stable branch.
Your instructions earlier in this thread as well as the link you referred to (How to add/edit a DDF on Home assistant using text editor? - #18 by bcutter) both mention something about a “hot reload” using a right-click when accessing “Edit DDF” in DeCONZ. Right-clicking that menu item does not seem to do anything. The dropdown menu just disappears. So I clicked “File->Hot reload Ctrl+R” in the DDF Editor window for ikea/fyrtur[…].json. Well, the status in the dialog persists at “Draft”.
What did I do wrong?
Maybe interesting side fact (at least for me): Upon rebooting the Raspi last night, the file in /usr/share/deCONZ/devices/ikea listed the “Draft” status again, after it showed “Gold” last time I took a look inside the container. So, apparently I has been able to change it using the webui prior to peeking inside the container, although that change did not persist (and apparently wasn’t picked up by DeCONZ?!).
General question: Seeing the Draft status in that file in the master branch on Github makes me wonder why not everyone using these Fyrtur roll-out covers would be affected. Or are they?
P.S. Also tried to pair it. Again, it’s just a hex number and “Edit DDF” shows “Draft”.