I set a picture with dark line at the bottom as kscreenlocker's image. I'd like to be able to move clock to that bottom part. I managed to hack share/plasma/look-and-feel/org.kde.breeze.desktop/contents/lockscreen/LockScreenUi.qml file to achieve what I want, but this isn't proper way to fix this.
Making the clock re-positionable is very unlikely to happen since it's a very niche use case, and, as, you've discovered, it's quite easy to adjust the code yourself. So ordinarily I'd say something like, "you can get a new lock screen theme more to your liking", but you actually can't since the lock screen theme is baked into the Plasma theme, unlike the SDDM login screen theme. So you would need to get a whole new Plasma theme, which seems like overkill. Maybe we should create a new store.kde.org category for lock screens?
The clock can now be hidden on the lock screen in the upcoming Plasma 5.18.