As if everything is not quite so simple and obvious according to the documentation
https://justin.ua/api/api_justin_documentation.pdfIt turns out that we can get information on the registers on the date of registration of the EH and get a list by the register on this date (p. 106).
Next, one by one, we can try to add the EH to one of the existing registries (if the registry has a EH in the "Planned" status, there will be an error) (p. 102).
If they couldn’t add it anywhere, they created a new registry and added EH there (p. 102).
If this algorithm is correct (I recommend discussing it with Justin technical support) - you can make an action that adds or removes EH from the registry - this action will take 6 hours.
Also, by analogy with New mail, you can make a block in mass changes, which will do the same as the action - it will take about 3 hours to implement (subject to the implementation of the above action).