After much faffing about, I have implemented the dynamic #OAuth2 client creation for #GoActivityPub services using the Client ID Metadata Document[1] that's been proposed as a replacement(?) for RFC7591 (Dynamic Client Registration Protocol).
The changes are in both the Authorization service and in the BOX #ActivityPub client to server helper.
[1] https://datatracker.ietf.org/doc/draft-ietf-oauth-client-id-metadata-document/
