Wew, good news:
I’ve got it working with official Syncthing by setting my folder marker to “IrfanView/.stsubfolder” which is an empty directory I’ve just created.
Settings > Advanced:
Content on file server:
Now Syncthing scans / syncs the folder while the ZFS pool as origin of data is available.
When I simulate a “pool interrupt” by making it suddenly unavailable by
zpool export pool0
Syncthing stops without advertising unwanted deletions to other nodes:
So if this will be constantly supported in the future, I can clearly recommend this approach as preferred over “symlink code hacks”.
Thumbs up from me that devs made this advanced option possible :-). 