The registry has a simple API that uses a Message-ID as the key to store arbitrary data (as long as it can be converted to a list for storage).
Store
valuein the extra data keykeyfor messageid.
If any extra entries are precious, their presence will make the registry keep the whole entry forever, even if there are no groups for the Message-ID and if the size limit of the registry is reached. By default this is just
(marks)so the custom registry marks are precious.