So here’s a new IETF I-D almost removes the need for this FEP, which is, myself and @aaronpk have started defining a specification for the OAuth WG that allows the client_id in OAuth 2 to be a URI to a document which describes the client (following current standards around dynamic client registration and various security concerns): OAuth Client ID Metadata Document
It is a draft, we are working on it, so language may change and there’s issues we probably need to address, but this solves needing dynamic client registration for federated systems, without conflating ActivityPub Actors and OAuth Clients / Applications.
The other thing in this is that the Client Identifier Metadata Document can include additional properties outside of those used for dynamic client registration / client metadata, which allows extensibility by others if they so desire (e.g., adding @context to a json-ld context like Solid might require)