I agree with the coarse permissions point, and I wouldn't bank on those services adding finer-grained scopes.

The idea in my draft is to do the attenuation and verification before the call reaches the service, enforced at the boundary, like the proxy setup you're describing. And the token wouldn't be a bearer token per se; there's proof of possession, and the constraints narrow at each hop and travel with the token, so the boundary can verify the chain itself rather than rely on a central authority. The design is inspired by macaroons and other capability-based access control work.

Full draft's here if you want to pick it apart: https://datatracker.ietf.org/doc/draft-niyikiza-oauth-attenu...