I can't find a way to preview changes, which will be performed by the batch tool chain in Digikam.

Closest, where I got, was opening batch tool and image editor side by side getting the result in image editor, then adding appropriate effect to the batch chain and repeating, in batch editor, changes which were done in image editor.

It does the job for effects like brightness changes, but if I wan't to change curves, for example, then there is no way to import saved curve profile from the image editor and it is impossible to repeat changes from the image editor by hand, because coordinates on the batch canvas are completely different from the image editor canvas.

So, if there isn't a way to preview image effect changes done by the batch tool, it renders the tool almost completely useless, for this task.

Ask on the digikam list - it's (very) active, and the dev(s) are very accommodating.

For anyone, who cares.
Here's the bug entry: https://bugs.kde.org/show_bug.cgi?id=321752
In the end there is a script, which should work as a separate solution.