This approach worked well until macOS 10.15 "Catalina" (and even works there for the main app). When previewing a file with Quick Look the related item file is not read. I assume the sandbox of Quick Look is not extended to the related item.
Before filing a RADAR, does anyone has a similar QL plugin and observing the same behavior?
Edit: If I perform the preview using qlmanage the related item is read...