I'm pretty sure almost all of these systems had something like that to hide things or DRM or protection or whatever. Even something like on newer OS's where if you read the disk one way you got one set of data then read the cd backwards it would give you something totally different because of the subdata correction data.
It will take time and effort and someone able and willing to slog through thousands of disks. It prob also means a v6 of the chd format as it is missing some bits needed for CDs/DVDs to be done 'just right'. Ideally able to take some of the existing formats and recreate them if needed.