@Frauke There is nothing now on the link you shared(
As I understood, the hash is used only as URI affix where URI represents DOI resolved by the centralized DOI gateway. Would you tell also about how the affix is formulated and hashed (maybe sharing the internal spec if it would be open)? I would be really appreciated studying your use case for implementation also of the openly reproducible PIDs and affix model based on them, like I described here: